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Java Programming Tutorial

OOP - Wrapping Up

The "static" Variables/Methods

You can apply modifier "static" to variables and methods (and inner classes).

• A static variable/method belongs to the class, and is shared by all instances. Hence, it is also called a class variable/method.
• On the other hand, a non-static variable/method (absence of keyword static) belongs to a specific instance of a class, also called an instance variable/method.

Each instance maintains its own storage. As the result, each instance variable/method has its own copy in the instances and not shared among different instances. To reference an instance variable/method, you need to identify the instance, and reference it via anInstanceName.aVariableName or anInstanceName.aMethodName().

A static variable/method has a single common memory location kept in the class and shared by all the instances. The JVM allocates static variable during the class loading. The static variable exists even if no instance is created. A static variable/method can be referenced via AClassName.aStaticVariableName or AClassName.aStaticMethodName(). It can also be referenced from any of its instances (but not recommended), e.g., instance1.aStaticVariableName or instance2.aStaticVariableName.

Non-static variables/methods belong to the instances. To use a non-static variable/method, an instance must first be constructed. On the other hand, static variables/methods belong to the class, they are "global" in nature. You need not construct any instance before using them.

The usage of static variables/methods are:

1. Provide constants and utility methods, such as Math.PI, Math.sqrt() and Integer.parseInt() which are public static and can be used directly through the class without constructing instances of the class.
2. Provide a "global" variable, which is applicable to all the instances of that particular class, for purposes such as counting the number of instances, resource locking among instances, and etc.

(I believe that the keyword "static" is used to signal it does not change among the instances. In C, a static variable in a function block does not change its value across multiple invocation. C++ extends static to denote class variables/methods.)

UML Notation: static variables/methods are underlined in the class diagram.

Examples

Counting the number of instance created - Instance variable won't work!

Suppose that we want to count the number of instances created. Using an instance variable doesn't work?!

public class Circle {
   public int count = 0;   // To count the number of instances created.
                           // Set to public to simplify access.
   private double radius;  // private variable
   public Circle(double radius) {  // constructor
      this.radius = radius;
      ++count;
   }

   // Test Driver
   public static void main(String[] args) {
      Circle c1 = new Circle(1.1);
      System.out.println("count is: " + c1.count);
      //count is: 1
      Circle c2 = new Circle(2.2);
      System.out.println("count is: " + c2.count);
      //count is: 1
      Circle c3 = new Circle(3.3);
      System.out.println("count is: " + c3.count);
      //count is: 1
   }
}

This is because count is an instance variable. Each instance maintains its own count. When a new instance is constructed, count is initialized to 0, then increment to 1 in the constructor.

Use a static variable to count the number of instances

We need to use a "static" variable, or class variable which is shared by all instances, to handle the count.

CircleWithStaticCount.java

public class CircleWithStaticCount {
   public static int count = 0;  // A static variable to count the number of instances created
                                 //   shared by all the instances
                                 // Set to public to simplify access
   private double radius; // An instance variable for each circle to maintain its own radius
   public CircleWithStaticCount(double radius) {  // constructor
      this.radius = radius;
      ++count;            // one more instance created
   }

   // Test Driver
   public static void main(String[] args) {
      CircleWithStaticCount c1 = new CircleWithStaticCount(1.1);
      System.out.println("count is: " + c1.count);
      //count is: 1
      // It is recommended to access static variable via the class instead of instances
      System.out.println("count is: " + CircleWithStaticCount.count);
      //count is: 1

      CircleWithStaticCount c2 = new CircleWithStaticCount(2.2);
      System.out.println("count is: " + CircleWithStaticCount.count);
      //count is: 2
      System.out.println("count is: " + c1.count);
      //count is: 2
      System.out.println("count is: " + c2.count);
      //count is: 2

      CircleWithStaticCount c3 = new CircleWithStaticCount(3.3);
      System.out.println("count is: " + CircleWithStaticCount.count);
      //count is: 3
      System.out.println("count is: " + c1.count);
      //count is: 3
      System.out.println("count is: " + c2.count);
      //count is: 3
      System.out.println("count is: " + c3.count);
      //count is: 3
   }
}

Using static variables/methods as "global" variables and "utility" methods

Another usage of "static" modifier is to provide "global" variables and "utility" methods that are accessible by other classes, without the need to create an instance of that providing class. For example, the class java.lang.Math composes purely public static variables and methods. To use the static variable in Math class (such as PI and E) or static methods (such as random() or sqrt()), you do not have to create an instance of Math class. You can invoke them directly via the class name, e.g., Math.PI, Math.E, Math.random(), Math.sqrt().

Non-static (instance) methods: Although from the OOP view point, each instance has its own copy of instance methods. In practice, the instances do not need their own copy, as methods do not have states and the implementation is exactly the same for all the instances. For efficiency, all instances use the copy stored in the class.

Within a class definition, a static method can access only static variables/methods. It cannot access non-static instance variables/methods, because you cannot identify the instance. On the other hand, an instance method can access static and non-static variables/methods. For example,

public class StaticTest {
   private static String msgStatic = "Hello from static";  // a static variable
   private String msgInstance = "Hello from non-static";   // a non-static instance variable
   public static void main(String[] args) {  // a static method
      System.out.println(msgStatic);
      //Hello from static
      //System.out.println(msgInstance);
      //compilation error: non-static variable xxx cannot be referenced from a static context
   }
}

If a class has only one single instance (known as singleton design pattern), it could be more efficient to use static variable/method for that particular one-instance class?!

static variable or methods cannot be hidden or overridden in the subclass as non-static.

The Static Initializer

A static initializer is a block of codes labeled static. The codes are executed exactly once, when the class is loaded. For example,

public class StaticInitializerTest {
   static int number;  // a static variable
   static {            // a static initializer block - run once when the class is loaded
      number = 88;
      System.out.println("running static initializer...");
   }

   public static void main(String[] args) {  // Test Driver
      System.out.println("running main()...");
      System.out.println("number is: " + number);
   }
   //running static initializer...
   //running main()...
   //number is: 88
}

During the class loading, JVM allocates the static variables and then runs the static initializer. The static initializer could be used to initialize static variables or perform an one-time tasks for the class.

The Class Loader

Every JVM has a built-in class loader (of type java.lang.ClassLoader) that is responsible for loading classes into the memory of a Java program. Whenever a class is referenced in the program, the class loader searches the classpath for the class file, loads the bytecode into memory, and instantiates a java.lang.Class object to maintain the loaded class.

The class loader loads a class only once, so there is only one java.lang.Class object for each class that used in the program. This Class object stores the static variables and methods.

During the class loading, the class loader also allocates the static variables, and invokes the explicit initializers and static initializers (in the order of appearance).

public class ClassLoaderTest {
   private static int number1 = 11;  // explicit initializer
   static {                          // static initializer
      System.out.println("running static initializer...");
      number1 = 99;
      number2 = 88;
      System.out.println("number1 is " + number1);
      //System.out.println("number2 is: " + number2);
      //compilation error: illegal forward reference
   }
   private static int number2 = 22;  // explicit initializer

   public static void main(String[] args) {  // Test Driver
      System.out.println("running main()...");
      System.out.println("in main(): number1 is " + number1 + ", number2 is: " + number2);
   }
   //running static initializer...
   //number1 is 99
   //running main()...
   //in main(): number1 is 99, number2 is: 22
}

The Instance Initializer

Similarly, you could use the so-called instance initializer, which runs during the instantiation process, to initialize an instance. Instance initializer is rarely-used. For example,

class Foo {
   int number;  // an instance variable
   {            // an instance initializer block - run once per instantiation
      System.out.println("running instance initializer...");
      number = 99;
   }
   public Foo() {  // Constructor
      System.out.println("running constructor...");
      System.out.println("number is: " + number);
   }
}

public class InstanceInitializerTest {
   public static void main(String[] args) {
      Foo f1 = new Foo();
      //running instance initializer...
      //running constructor...
      //number is: 99
      Foo f2 = new Foo();
      //running instance initializer...
      //running constructor...
      //number is: 99
   }
}

The "Instantiation" Process

The sequence of events when a new object is instantiated via the new operator (known as the instantiation process) is as follows:

1. JVM allocates memory for the instance in the heap.
2. JVM initializes the instance variables to their assigned values or default values.
3. JVM invokes the constructor.
4. The first statement of the constructor is always a call to its immediate superclass' constructor. JVM invokes the selected superclass' constructor.
5. JVM executes the instance initializers in the order of appearance.
6. JVM executes the body of the constructor.
7. The new operator returns a reference to the new object.

For example,

class Foo {
   public int number1 = 11;  // explicit initializer
   {                         // instance initializer
      number1 = 33;
      number2 = 44;
   }
   public int number2 = 22;  // explicit initializer

   public Foo() {
      System.out.println("running Foo()...");
   }

   public Foo(int number1, int number2) {  // Constructor
      System.out.println("running Foo(int, int)...");
      this.number1 = number1;  // run after initializers
      this.number2 = number2;
   }
}

public class InstantiationTest {
   public static void main(String[] args) {
      Foo f1 = new Foo();
      //running Foo()...
      System.out.println("number1 is " + f1.number1 + ", number2 is " + f1.number2);
      //number1 is 33, number2 is 22

      Foo f2 = new Foo(55, 66);
      //running Foo(int, int)...
      System.out.println("number1 is " + f2.number1 + ", number2 is " + f2.number2);
      //number1 is 55, number2 is 66
   }
}

The "final" Class/Variable/Method

You can declare a class, a variable or a method final.

• A final class cannot be sub-classed (or extended).
• A final method cannot be overridden in the subclasses.
• A final variable cannot be re-assigned a new value.

final Variables of Primitive Type vs. Reference Type

• A final variable of primitive type is a constant, whose value cannot be changed. A "public static final" variable of primitive type is a global constant, whose value cannot be changed. For example,

  // class java.lang.Math
  public static final double PI = 3.141592653589793;
  public static final double E = 2.718281828459045;
  // class java.lang.Integer
  public static final int MAX_VALUE = 2147483647;
  public static final int MIN_VALUE = -2147483648;
  public static final int SIZE = 32;

• A final variable of a reference type (e.g., an instance of a class or an array) cannot be re-assigned a new reference. That is, you can modify the content of the instance, but cannot re-assign the variable to another instance. For example,

  public class FinalReferenceTest {
     public static void main(String[] args) {
        final StringBuffer sb = new StringBuffer("hello");  // final reference of mutable object
        sb.append(", world");   // can change the contents of the referenced object
        System.out.println(sb);
        //hello, world
  
        //sb = new StringBuffer("world peace");
        //compilation error: cannot assign a value to final variable xxx
  
        final int[] numbers = {11, 22, 33};  // a final array reference
        numbers[0] = 44;
        System.out.println(java.util.Arrays.toString(numbers));
        //[44, 22, 33]
     }
  }

final vs. abstract

final is opposite to abstract. A final class cannot be extended; while an abstract class must be extended and the extended class can then be instantiated. A final method cannot be overridden; while an abstract method must be overridden to complete its implementation. [abstract modifier is applicable to class and method only.]

Package, Import, Classpath & JAR

If I have a class called Circle and you also have a class called Circle. Can the two Circle classes co-exist or even be used in the same program? The answer is yes, provided that the two Circle classes are placed in two different packages.

A package, like a library, is a collection of classes, and other related entities such as interfaces, errors, exceptions, annotations, and enums.

UML Notation: Packages are represented in UML notation as tabbed folders, as illustrated.

Package name (e.g., java.util) and classname (e.g., Scanner) together form the so-called fully-qualified name in the form of packagename.classname (e.g., java.util.Scanner), which unambiguously identifies a class.

Packages are used for:

1. Organizing classes and related entities.
2. Managing namespaces - Each package is a namespace.
3. Resolving naming conflicts. For example, com.zzz.Circle and com.yyy.Circle are treated as two distinct classes. Although they share the same classname Circle, they belong to two different packages: com.zzz and com.yyy. These two classes can co-exist and can even be used in the same program via the fully-qualified names.
4. Access control: Besides public and private, you can grant access of a class/variable/method to classes within the same package only.
5. Distributing Java classes: All entities in a package can be combined and compressed into a single file, known as JAR (Java Archive) file, for distribution.

Package Naming Convention

A package name is made up of the reverse of the domain Name (to ensure uniqueness) plus your own organization's project name separated by dots. Package names are in lowercase. For example, suppose that your Internet Domain Name is "zzz.com", you can name your package as "com.zzz.project1.subproject2".

The prefix "java" and "javax" are reserved for the core Java packages and Java extensions, e.g., java.lang, java.util, and java.net, javax.net.

Package Directory Structure

The "dots" in a package name correspond to the directory structure for storing the class files. For example, the com.zzz.Cat is stored in directory "...\com\zzz\Cat.class" and com.yyy.project1.subproject2.Orange is stored in directory "...\com\yyy\project1\subproject2\Orange.class", where "..." denotes the base directory of the package.

JVM can locate your class files only if the package base directory and the fully-qualified name are given. The package base directory is provided in the so-called classpath (to be discussed later).

The "dot" does not mean sub-package (there is no such thing as sub-package). For example, java.awt and java.awt.event are two distinct packages. Package java.awt is kept in "...\java\awt"; whereas package java.awt.event is stored in "...\java\awt\event".

module

JDK 9 introduces a hierarchical level called "module" on top of packages, which will not be covered in this article.

The "import" Statement

There are two ways to reference a class in your source codes:

1. Use the fully-qualified name in the form of packagename.classname (such as java.util.Scanner). For example,

   public class ScannerNoImport {
      public static void main(String[] args) {
         // Use fully-qualified name in "ALL" the references
         java.util.Scanner in = new java.util.Scanner(System.in);
         System.out.print("Enter a integer: ");
         int number = in.nextInt();
         System.out.println("You have entered: " + number);
      }
   }

   Take note that you need to use the fully-qualified name for ALL the references to the class. This is clumpy!
2. Add an "import fully-qualified-name" statement at the beginning of the source file. You can then use the classname alone (without the package name) in your source codes. For example,

   import java.util.Scanner;
   public class ScannerWithImport {
      public static void main(String[] args) {
         // Package name can be omitted for an imported class
         // Java compiler searches the import statements for the fully-qualified name
         Scanner in = new Scanner(System.in);  // classname only
         System.out.print("Enter a integer: ");
         int number = in.nextInt();
         System.out.println("You have entered: " + number);
      }
   }

The compiler, when encounter a unresolved classname, will search the import statements for the fully-qualified name.

The import statement provides us a convenient way for referencing classes without using the fully-qualified name. "Import" does not load the class, which is carried out by the so-called class loader at runtime. It merely resolves a classname to its fully-qualified name, or brings the classname into the namespace. "Import" is strictly a compiled-time activity. The Java compiler replaces the classnames with their fully-qualified names, and removes all the import statements in the compiled bytecode. There is a slight compile-time cost but no runtime cost.

The import statement(s) must be placed after the package statement but before the class declaration. It takes the following syntax:

import packagename.classname;
import packagename.*

You can import a single class in an import statement by providing its fully-qualified name, e.g.,

import java.util.Scanner;  // import the class Scanner in package java.util
import java.awt.Graphics;  // import the class Graphics in package java.awt

You can also import all the classes in a package using the wildcard *. The compiler will search the entire package to resolve classes referenced in the program. E.g.,

import java.util.*;       // import all classes in package java.util
import java.awt.*;        // import all classes in package java.awt
import java.awt.event.*;  // import all classes in package java.awt.event

Using wildcard may result in slightly fewer source lines. It has no impact on the resultant bytecode. It is not recommended as it lacks clarity and it may lead to ambiguity if two packages have classes of the same names.

The Java core language package java.lang is implicitly imported to every Java program. Hence no explicit import statements are needed for classes inside the java.lang package, such as System, String, Math, Integer and Object.

There is also no need for import statements for classes within the same package.

Take note that the import statement does not apply to classes in the default package.

The "import static" Statement (JDK 1.5)

Prior to JDK 1.5, only classes can be "imported" - you can omit the package name for an imported class. In JDK 1.5, the static variables and methods of a class can also be "imported" via the "import static" declaration - you can omit the classname for an imported static variable/method. For example:

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import static java.lang.System.out; // import static variable "out" of "System" class
import static java.lang.Math.*;     // import "ALL" static variables/methods in "Math" class
 
public class TestImportStatic {
   public static void main(String[] args) {
      // Classname can be omitted for imported static variables/methods
      out.println("Hello, PI is " + PI);
      out.println("Square root of PI is " + sqrt(PI));
   }
}

The import static statement takes the following syntax:

import static packagename.classname.staticVariableName;
import static packagename.classname.staticMethodName;
import static packagename.classname.*;   // wildcard * denotes all static variables/methods of the class

Take note that import and import static statements does not apply to classes/members in the default package.

Creating Packages

To put a class as part of a package, include a package statement before the class definition (as the FIRST statement in your program). For example,

package com.zzz.test;
 
public class HelloPackage {
   public static void main(String[] args) {
      System.out.println("Hello from a package...");
   }
}

You can create and use package in IDE (such as Eclipse/NetBeans) easily, as the IDE takes care of the details. You can simply create a new package, and then create a new class inside the package.

Compiling Classes in Package

To compile classes in package using JDK, you need to use "-d" flag to specify the destination package base directory, for example,

// Set the current working directory to the directory containing HelloPackage.java
> javac -d e:\myproject HelloPackage.java

The "-d" option instructs the compiler to place the class file in the given package base directory, as well as to create the necessary directory structure for the package. Recall that the dot '.' in the package name corresponds to sub-directory structure. The compiled bytecode for com.zzz.test.HelloPackage will be placed at "e:\myproject\com\zzz\test\HelloPackage.class"

Running Classes in Package

To run the program, you need to set your current working directory at the package base directory (in this case "e:\myproject"), and provide the fully-qualify name:

// Set the current working directory to the package base directory
e:\myproject> java com.zzz.test.HelloPackage

It is important to take note that you shall always work in the package base directory and issue fully-qualified name.

As mentioned, if you use an IDE, you can compile/run the classes as usual. IDE will take care of the details.

The Default Unnamed Package

So far, all our examples do not use a package statement. These classes belong to a so-called default unnamed package. Use of the default unnamed package is not recommended should be restricted to toy programs only, as they cannot be "imported" into another application. For production, you should place your classes in proper packages.

Java Archive (JAR)

An Java application typically involves many classes. For ease of distribution, you could bundles all the class files and relevant resources into a single file, called JAR (Java Archive) file.

JAR uses the famous "zip" algorithm for compression. It is modeled after Unix's "tar" (Tape ARchive) utility. You can also include your digital signature (or certificate) in your JAR file for authentication by the recipients.

JDK provides an utility called "jar" to create and manage JAR files. For example, to create a JAR file, issue the following command:

// To create a JAR file from c1 ... cn classes (c:create, v:verbose, f:filename):
> jar cvf myjarfile.jar c1.class ... cn.class

Example

To place the earlier class com.zzz.test.HelloPackage (and possible more related classes and resources) in a JAR file called hellopackage.jar:

// Set the current working directory to the package base directory (i.e., e:\myproject)
e:\myproject> jar cvf hellopackage.jar com\zzz\test\HelloPackage.class
added manifest
adding: com/zzz/test/HelloPackage.class(in = 454) (out= 310)(deflated 31%)

Read "Java Archive (JAR)" for more details.

Classpath - Locating Java Class Files

Java allows you to store your class files anywhere in your file system. To locate a class, you need to provide the package base directory called classpath (short for user class search path) and the fully-qualified name. For example, given that the package base directory is e:\myproject, the class com.zzz.test.HelloPackage can be found in e:\myproject\com\zzz\test\HelloPackage.class.

When the Java compiler or runtime needs a class (given its fully-qualified name), it searches for it from the classpath. You could specify the classpath via the command-line option -cp (or -classpath); or the environment variable CLASSPATH.

A classpath may contain many entries (separated by ';' in Windows or ':' in Unixes/Mac). Each entry shall be a package base directory (which contains many Java classes), or a JAR file (which is a single-file archives of many Java classes).

Example on Package, Classpath and JAR

In this example, we shall kept the source files and class files in separate directories - "src" and "bin" - for ease of distribution minus the source.

com.zzz.geometry.Circle

Let's create a class called Circle in package com.zzz.geometry. We shall keep the source file as d:\zzzpackages\src\com\zzz\geometry\Circle.java and the class file in package base directory of d:\zzzpackages\bin.

package com.zzz.geometry;
public class Circle {      // save as d:\zzzpackages\src\com\zzz\geometry\Circle.java
   public String toString() {
      return "This is a Circle";
   }
}

To compile the Circle class, use javac with -d option to specify the destination package base directory.

// Set current working directory to source file (d:\zzzpackages\src\com\zzz\geometry)
> javac -d d:\zzzpackages\bin Circle.java
// Output class file is d:\zzzpackages\bin\com\zzz\geometry\Circle.class

com.zzz.geometry.Cylinder

Next, create a class called Cylinder in the same package (com.zzz.geometry) that extends Circle.

package com.zzz.geometry;
public class Cylinder extends Circle {  // save as d:\zzzpackages\src\com\zzz\geometry\Cylinder.java
   public String toString() {
      return "This is a Cylinder";
   }
}

No import statement for Circle is needed in Cylinder, because they are in the same package.

To compile the Cylinder class, we need to provide a classpath to the Circle class via option -cp (or -classpath), because Cylinder class references Circle class.

// Set current working directory to source file (d:\zzzpackages\src\com\zzz\geometry)
> javac -d d:\zzzpackages\bin -cp d:\zzzpackages\bin Cylinder.java
// Output class file is d:\zzzpackages\bin\com\zzz\geometry\Cylinder.class

com.yyy.animal.Cat

Create another class called Cat in another package (com.yyy.animal). We shall keep the source file as d:\yyypackages\src\com\yyy\animal\Cat.java and the class file in package base directory of d:\yyypackages\bin.

package com.yyy.animal;
public class Cat {   // save as d:\yyypackages\src\com\yyy\animal\Cat.java
   public String toString() {
      return "This is a Cat!";
   }
}

Again, use -d option to compile the Cat class. No classpath needed as the Cat class does not reference other classes.

// Set current working directory to source file (d:\yyypackages\src\com\yyy\animal)
> javac -d d:\yyypackages\bin Cat.java
// Output class file is d:\yyypackages\bin\com\yyy\animal\Cat.class

myTest.test

We shall write a Test class (in package myTest) to use all the classes. We shall keep the source file as d:\testpackages\src\mytest\Test.java and the class file in package base directory of d:\testpackages\bin.

package mytest;
 
import com.zzz.geometry.Circle;
import com.zzz.geometry.Cylinder;
import com.yyy.animal.Cat;
 
public class Test {   // save as d:\testpackages\src\mytest\Test.java
   public static void main(String[] args) {
      Circle circle = new Circle();
      System.out.println(circle);
      Cylinder cylinder = new Cylinder();
      System.out.println(cylinder);
      Cat cat = new Cat();
      System.out.println(cat);
   }
}

To compile the Test class, we need -d option to specify the destination and -cp to specify the package base directories of Circle and Cylinder (d:\zzzpackages\bin) and Cat (d:\yyypackages\bin).

// Set current working directory to source file (d:\testpackages\src\mytest)
> javac -d d:\testpackages\bin -cp d:\zzzpackages\bin;d:\yyypackages\bin Test.java
// Output class file is d:\testpackages\bin\mytest\Test.class

To run the myTest.Test class, set the current working directory to the package base directory of mytest.Test (d:\testpackages\bin) and provide classpath for Circle and Cylinder (d:\zzzpackages\bin), Cat (d:\yyypackages\bin) and the current directory (for mytest.Test).

// Set current working directory to package base directory (d:\testpackages\bin)
> java -cp .;d:\zzzpackages\bin;d:\yyypackages\bin mytest.Test

Jarring-up com.zzz.geometry package

Now, suppose that we decided to jar-up the com.zzz.geometry package into a single file called geometry.jar (and kept in d:\jars):

// Set current working directory to package base directory (d:\zzzpackages\bin)
// 'c' to create, 'v' for verbose, 'f' follows by jar filename
> jar cvf d:\jars\geometry.jar com\zzz\geometry\*.class
added manifest
adding: com/zzz/geometry/Circle.class(in = 300) (out= 227)(deflated 24%)
adding: com/zzz/geometry/Cylinder.class(in = 313) (out= 228)(deflated 27%)
// Output is d:\jars\geometry.jar
 
// OR
// Set current working directory to package base directory (d:\zzzpackages\bin)
// jar the current directory (.) and its sub-directories
> jar cvf d:\jars\geometry.jar .
added manifest
adding: com/(in = 0) (out= 0)(stored 0%)
adding: com/zzz/(in = 0) (out= 0)(stored 0%)
adding: com/zzz/geometry/(in = 0) (out= 0)(stored 0%)
adding: com/zzz/geometry/Circle.class(in = 300) (out= 227)(deflated 24%)
adding: com/zzz/geometry/Cylinder.class(in = 313) (out= 228)(deflated 27%)

To run mytest.Test with the JAR file, set the classpath to the JAR file (classpath accepts both directories and JAR files).

// Set current working directory to package base directory (d:\testpackages\bin)
> java -cp .;d:\jars\geometry.jar;d:\yyypackages\bin mytest.Test

Separating Source Files and Classes

For ease of distribution (without source files), the source files and class files are typically kept in separate directories.

1. Eclipse keeps the source files under "src", class files under "bin", and jar files and native libraries under "lib".
2. NetBeans keeps the source files under "src", class files under "build\classes", jar files and native libraries under "build\lib".

Two Classes of the Same Classname?

Suppose that we have two Circle classes in two different packages, can we use both of them in one program? Yes, however, you need to use fully-qualified name for both of them. Alternatively, you may also import one of the classes, and use fully-qualified name for the other. But you cannot import both, which triggers a compilation error.

How JVM Find Classes

Reference: JDK documentation on "How classes are found".

To locate a class (given its fully-qualified name), you need to locate the base directory or the JAR file.

The JVM searches for classes in this order:

1. Java Bootstrap classes: such as "rt.jar" (runtime class), "i18n.jar" (internationalization class), charsets.jar, jre/classes, and others.
2. Java Standard Extension classes: JAR files located in "$JDK_HOME\jre\lib\ext" directory (for Windows and Ubuntu); "/Library/Java/Extensions" and "/System/Library/Java/Extensions" (for Mac). The location of Java's Extension Directories is kept in Java's System Property "java.ext.dirs".
3. User classes.

The user classes are searched in this order:

1. The default ".", i.e., the current working directory.
2. The CLASSPATH environment variable, which overrides the default.
3. The command-line option -cp (or -classpath), which overrides the CLASSPATH environment variable and default.
4. The runtime command-line option -jar, which override all the above.

The JVM puts the classpath is the system property java.class.path. Try running the following line with a -cp option and without -cp (which uses CLASSPATH environment variable) to display the program classpath:

System.out.println(System.getProperty("java.class.path"));

javac|java's command-line option -classpath or -cp

I have demonstrated the command-line option -classpath (or -cp) in the earlier example.

The CLASSPATH Environment Variable

Alternatively, you could also provide your classpath entries in the CLASSPATH environment variable. Take note that if CLASSPATH is not set, the default classpath is the current working directory. However, if you set the CLASSPATH environment variable, you must include the current directory in the CLASSPATH, or else it will not be searched.

Read "Environment Variables (PATH, CLASSPATH, JAVA_HOME)" for more details about CLASSPATH environment variable.

It is recommended that you use the -cp (-classpath) command-line option (customized for each of your applications), instead of setting a permanent CLASSPATH environment for all the Java applications. IDE (such as Eclipse/NetBeans) manages -cp (-classpath) for each of the applications and does not rely on the CLASSPATH environment.

More Access Control Modifiers – protected and default package-private

Java has four access control modifiers for class/variable/method. Besides the public (available to all outside classes) and private (available to this class only), they are two modifiers with visibility in between public and private:

• protected: available to all classes in the same package and the subclasses derived from it.
• default (package-private): If the access control modifier is omitted, by default, it is available to classes in the same package only. This is also called package-private accessibility.

Java Source File

A Java source file must have the file type of ".java". It can contain at most one top-level public class, but may contain many non-public classes (not recommended). The file name shall be the same as the top-level public classname.

The source file shall contain statements in this order:

1. Begins with one optional package statement. If the package statement is omitted, the default package (.) is used. Use of default package is not recommended for production.
2. Follows by optional import or import static statement(s).
3. Follows by class, interface or enum definitions.

Each class, interface or enum is compiled into its own ".class" file.

The top-level class must be either public or default. It cannot be private (no access to other classes including JVM?!) nor protected (meant for member variables/methods accessible by subclasses), which triggers compilation error "modifier private|protected not allowed here".

Dissecting the Hello-world

Let us re-visit the "Hello-world" program, which is reproduced below:

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public class Hello {
   public static void main(String[] args) {
      System.out.println("hello, world");
   }
}

• The class Hello is declared public so that it is accessible by any other classes. In this case, the JRE needs to access the Hello class to run the main().
  Try declaring the Hello class private/protected/package and run the program.

  private class Hello { ... }
  //compilation error: modifier private not allowed here
  
  protected class Hello { ... }
  //compilation error: modifier protected not allowed here
  
  class Hello { ... }  // default of package access
  //okay

  private and protected are not allowed for outer class. package is fine and JRE can also run the program?! What is the use of a private class, which is not accessible to others? I will explain the usage of private class later in the so-called inner class.
• Similarly, the main() method is declared public, so that JRE can access and invoke the method.
  Try declaring the main() method private/protected/package.

     private|protected|package static void main(String[] args) { ... }
     //no compilation error
     //runtime error: Main method not found in class Hello

  You can compile the main() with private/protected/package, but cannot run the main() method.
• The main() method is declared static. Remember that a static variable/method belongs to the class instead of a particular instance. There is no need to create an instance to use a static variable/method. A static method can be invoked via the classname, in the form of aClassName.aStaticMethodName(). JRE can invoke the static main() method, by calling Hello.main() from the class directly. Note that we did not create any instance of the Hello class.
  Try omitting the static keyword and observe/explain the error message.

     public void main(String[] args) { ... }  // missing static - an instance method
     //no compilation error
     //runtime error: Main method not found in class Hello

• The main() method takes an argument of a String array, corresponding to the command-line arguments supplied by the user, performs the program operations, and return void (or nothing) to the JRE.
  Try omitting the argument (String[] args) from the main() method.

     public static void main() { ... }  // missing String[] argument - another overloaded version of main()
     //no compilation error
     //runtime error: Main method not found in class Hello

  You can compile, but JRE cannot find the matching main(String[]).
• In C language, the signature of main() function is:

  main(int argc, char *argv[]) { ...... }

  Two parameters are needed for the command-line argument - int argc to specify the number of arguments and the string-array argv to keep the arguments. In Java, only one parameter - a String array is needed. This is because Java array contains the length internally, and the number of arguments can be retrieved via args.length.
  Furthermore, in C, the name of the program is passed as the first command-line argument. In Java, the program name is not passed, as the class name is kept within the object. You can retrieve the class name via this.getClass().getName().

System.out.println()

If you check the JDK API specification, you will find that:

• "System" is a class in the package java.lang.
• "out" is a static public variable of the class java.lang.System.
• "out" is an instance of class "java.io.PrintStream".
• The class java.io.PrintStream provides a public method called "println()".

The figure illustrate the classes involved in System.out.println().

Take note that each of the dot (.) opens a 3-compartment box!!!

Example

As an example, the reference "A.b.c().d.e()" can be interpreted as follows:

• "A" is a class.
• "b" is a public static variable of class "A" (because it is referenced via the classname).
• The variable "b" belongs to a class say "X".
• The class "X" provides a public method "c()".
• The "c()" method returns an instance "y" of class say "Y".
• The "Y" class has a variable (static or instance) called "d".
• The variable "d" belongs to a class say "Z".
• The class "Z" provides a public method called "e()".

Nested and Inner Classes

Read "Nested and Inner Classes".

More on Variables and References

Types of Variables

The type of a variable determines what kinds of value the variable can hold and what operations can be performed on the variable. Java is a "strong-type" language, which means that the type of the variables must be known at compile-time.

Java has three kinds of types:

1. Primitive type: There are eight primitive types in Java: byte, short, int, long, float, double, char, and boolean. A primitive-type variable holds a simple value.
2. Reference type: Reference types include class, interface, enum and array. A reference-type variable holds a reference to an object or array.
3. A special null type, holding a special null reference. It could be assigned to a reference variable that does not reference any object.

A primitive variable holds a primitive value (in this storage). A reference variable holds a reference to an object or array in the heap, or null. A references variable can hold a reference of the type or its sub-type (polymorphism). The value null is assigned to a reference variable after it is declared. A reference is assigned after the instance is constructed. An object (instance) resides in the heap. It must be accessed via a reference.

Java implicitly defines a reference type for each possible array type - one for each of the eight primitive types and an object array.

Scope & Lifetime of Variables

The scope of a variable refers to the portion of the codes that the variable can be accessed. The lifetime refers to the span the variable is created in the memory until it is destroyed (garbage collected). A variable may exist in memory but not accessible by certain codes.

Java supports three types of variables of different lifetimes:

Automatic Variable (or Local Variable): Automatic Variables include method's local variables and method's parameters. Automatic variables are created on entry to the method and are destroyed when the method exits. The scope of automatic variables of a method is inside the block where they are defined. Local variable cannot have access modifier (such as private or public). The only modifier applicable is final.

For example,

public class AutomaticVariableTest {
   public static void main(String[] args) {  // scope of method parameter args is within this method
      for (int i = 0; i < 10; ++i) {  // scope of i is within the for-loop
         System.out.println(i);
      }
      //System.out.println(i);  // i has gone out of scope
      //compilation error: cannot find symbol

      int j = 0;  // scope of j is within the method, from this statement onwards (forward reference)
      for (j = 0; j < 10; ++j) {
         System.out.println(j);
      }
      System.out.println(j);  // okay

      int k = 1;
      do {
         int x = k*k;     // scope of x is within the block
         ++k;
         System.out.println(k);
      } while (x < 100);  //compilation error: cannot find symbol
   }
}

Member variable (or Instance variable) of a Class: A member variable of a class is created when an instance is created, and it is destroyed when the object is destroyed (garbage collected).

Static variable (or Class variable) of a Class: A static variable of a class is created when the class is loaded (by the JVM's class loader) and is destroyed when the class is unloaded. There is only one copy for a static variable, and it exists regardless of the number of instances created, even if the class is not instantiated. Take note that static variables are created (during class loading) before instance variables (during instantiation).

Variable Initialization

All class member and static variables that are not explicitly assigned a value upon declaration are assigned a default initial value:

• "zero" for numeric primitive types: 0 for int, byte, short and long, 0.0f for float, 0.0 for double;
• '\u0000' (null character) for char;
• false for boolean;
• null for reference type (such as array and object).

You can use them without assigning an initial value.

Automatic variables are not initialized, and must be explicitly assigned an initial value before it can be referenced. Failure to do so triggers a compilation error "variable xxx might not have been initialized".

Array Initializer

Array's elements are also initialized once the array is allocated (via the new operator). Like member variables, elements of primitive type are initialized to zero or false; while reference type are initialized to null. (Take note that C/C++ does not initialize array's elements.)

For example,

String[] strArray = new String[2];
for (String str: strArray) {
   System.out.println(str);
}
//null
//null

You can also use the so-called array initializer to initialize the array during declaration. For example,

int[] numbers = {11, 22, 33};
String[] days = {"Monday", "Tuesday", "Wednesday"};
Circle[] circles = {new Circle(1.1), new Circle(2.2), new Circle(3.3)};
float[][] table = {{1.1f, 2.2f, 3.3f}, {4.4f, 5.5f, 6.6f}, {7.7f, 8.8f, 9.9f}};
int[][] data = {{1, 4, 8}, {2, 3}, {4, 8, 1, 5}};

Stack/Heap and Garbage Collector

Where Primitives and Objects Live?

Primitive types, such as int and double, are created in the program's method stack during compiled time for efficiency (less storage and fast access). Java's designer retained primitives in a object-oriented language for its efficiency.

Reference types, such as objects and arrays, are created in the "heap" at runtime (via the new operator), and accessed via a reference. Heap is less efficient as stack, as complex memory management is required to allocate, manage and release storage.

For automatic variable of reference type: the reference is local (allocated in the method stack), but the object is allocated in the heap.

Stack and heap are typically located at the opposite ends of the data memory, to facilitate expansion.

Object References

When a Java object is constructed via the new operator and constructor, the constructor returns a value, which is a bit pattern that uniquely identifies the object. This value is known as the object reference.

In some JVM implementations, this object reference is simply the address of the object in the heap. However, the JVM specification does not specify how the object reference shall be implemented as long as it can uniquely identify the object. Many JVM implementations use so-called double indirection, where the object reference is the address of an address. This approach facilitates the garbage collector (to be explained next) to relocate objects in the heap to reduce memory fragmentation.

Objects are created via the new operator and the constructor. The new operator:

1. creates a new instance of the given class, and allocate memory dynamically from the heap;
2. calls one of the overloaded constructors to initialize the object created; and
3. returns the reference.

For primitives stored in the stack, compiler can determine how long the item lasts and destroy it once it is out of scope. For object in heap, the compiler has no knowledge of the creation and lifetime of the object.

In C++, you must destroy the heap's objects yourself in your program once the objects are no longer in use (via delete operator). Otherwise, it leads to a common bug known as "memory leak" - the dead objects pile-up and consume all the available storage. On the other hand, destroying an object too early, while it is still in use, causes runtime error. Managing memory explicitly is tedious and error prone, although the programs can be more efficient.

In Java, you don't have to destroy and de-allocate the objects yourself. JVM has a built-in process called garbage collector that automatically releases the memory for an object when there is no more reference to that object. The garbage collector runs in a low priority thread.

An object is eligible for garbage collection when there is no more references to that object. Reference that is held in a variable is dropped when the variable has gone out of its scope. You may also explicitly drop an object reference by setting the object reference to null to signal to the garbage collector it is available for collection. However, it may or may not get garbage collected because there is no guarantee on when the garbage collector will be run or it will be run at all. The garbage collector calls the object's destructor (a method called finalize()), if it is defined, before releasing the memory back to the heap for re-use.

If a new reference is assigned to a reference variable (e.g., via new and constructor), the previous object will be available for garbage collection if there is no other references.

System.gc() & Runtime.gc()

You can explicitly ask for garbage collection by calling static methods System.gc() or Runtime.gc(). However, the behavior of these methods is JVM dependent. Some higher priority thread may prevent garbage collector from being run. You cannot rely on the gc() methods to perform garbage collection as the JVM specification merely states that "calling this method suggests that the Java Virtual Machine expends effort toward recycling unused objects". So the critical question "When the storage is recovered?" cannot be answered in Java.

Pitfalls of Java

Java's garbage collector frees you from worrying about memory management of objects (no more free or delete like C/C++) so that you can focus on more productive works. It also insure against so called "memory leak" (i.e., used objects were not de-allocated from memory and slowly fill up the precious memory space); or releasing object too early which results in runtime error. These are common problems in C/C++ programs.

However, garbage collector does has its drawbacks:

1. Garbage collector consumes computational resources and resulted in runtime overhead.
2. The rate of execution is not guarantee and can be inconsistent. This is because JVM specification does not spell out when and how long the garbage collector should be run. This may have an impact on real-time programs, when a response is expected within a certain time, which cannot be interrupted by the garbage collector.

Many programmers prefer to use C++ for game programming and animation, as these programs could create millions of objects in a short span. Managing memory efficiently is critical, instead of relying on garbage collector.

There are some (imperfect) solutions to memory management in Java, e.g.,

1. Pre-allocate and re-use the objects, instead of creating new objects. This requires effort from programmers.
2. The author of "jBullet", which is a Java port of the famous Collision Physics library "Bullet Physics", created a library called jStackAlloc, which allocates objects on the method's stack instead of program heap. This improves real-time performance by reducing the frequency of garbage collection.

This solution shall remain imperfect until the Java designers decided to allow programmers to manage the storage, which is not likely.

More on Methods

Passing Arguments into Methods - By Value vs. By Reference

Recall that a method receives arguments from the caller, performs operations defined in the method body, and returns a piece of result or void to the caller.

To differentiate the parameters inside and outside the method, we have:

• Actual parameters (or arguments): The actual values passed into the method and used inside the method.
• Formal parameters (or method parameters): The placeholders used in the method definition, which are replaced by the actual parameters when the method is invoked.

For example:

public static double getArea(double radius) {  // radius is called formal parameter
   return radius * radius * Math.PI;
}
public static void main(String[] args) {
   double r = 1.2;
   System.out.println(getArea(r));    // these are the actual parameter (or argument)
   System.out.println(getArea(3.4));
}

In the above method definition, radius is a parameter placeholder or formal parameter. If we invoke the method with a variable r with value of 1.2, i.e., getArea(r), r=1.2 is the actual parameter.

Passing Primitive-type Argument into Method - Pass-by-Value

If the argument is a primitive type (e.g., int or double), a copy of identical value is created and passed into the method. The method operates on the cloned copy. It does not have access to the original copy. If the value of the argument is changed inside the method, the original copy is not affected. This is called pass-by-value (passing a cloned value into the method).

For example,

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public class TestPassingPrimitive {
   public static void main(String[] args) {
      int number = 10;              // primitive type
      System.out.println("In caller, before calling the method, the value is: " + number);
      aMethodWithPrimitive(number); // invoke method
      System.out.println("In caller, after calling the method, the value is: " + number);
   }
 
   public static void aMethodWithPrimitive(int number) {
      System.out.println("Inside method, before operation, the value is " + number);
      ++number;  // change the parameter
      System.out.println("Inside method, after operation, the value is " + number);
   }
}

In caller, before calling the method, the value is: 10
Inside method, before operation, the value is 10
Inside method, after operation, the value is 11
In caller, after calling the method, the value is: 10

Although the variables are called number in the caller as well as in the method's formal parameter, they are two different copies with their own scope.

Passing Reference-Type Argument into Method - Also Pass-by-Value

If the argument is a reference type (e.g., an array or an instance of a class), a copy of the reference is created and passed into the method. Since the caller's object and the method's parameter have the same reference, if the method changes the member variables of the object, the changes are permanent and take effect outside the method.

For example,

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public class TestParameterReference {
   public static void main(String[] args) {
      StringBuffer sb = new StringBuffer("Hello");
      System.out.println("In caller, before calling the method, the object is \"" + sb + "\"");
      aMethodOnReference(sb);  // invoke method with side-effect
      System.out.println("In caller, after calling the method, the object is \"" + sb + "\"");
   }
 
   public static void aMethodOnReference(StringBuffer sb) {
      System.out.println("Inside method, before change, the object is \"" + sb + "\"");
      sb.append(", world");  // change parameter
      System.out.println("Inside method, after change, the object is \"" + sb + "\"");
   }
}

In caller, before calling the method, the object is "Hello"
Inside method, before change, the object is "Hello"
Inside method, after change, the object is "Hello, world"
In caller, after calling the method, the object is "Hello, world"

If a method affect values outside the method itself other than the value returned, we say that the method has side-effect. Side effects may not be obvious by reading the method's codes, and must be handled with extreme care, and should be avoided if feasible. Proper comments should be provided in the method's header.

Re-assigning the Reference inside the Method

Since a copy of the reference is passed into the method, if the method re-assigns the reference to the argument, the caller's object and the argument will not have the same reference. Change in the argument will not be reflected in the caller's object.

For example,

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public class TestParameterReferenceReassign {
   public static void main(String[] args) {
      StringBuffer sb = new StringBuffer("Hello");
      System.out.println("In caller, before calling the method, the object is \"" + sb + "\"");
      aMethodOnReference(sb);  // invoke method with side-effect
      System.out.println("In caller, after calling the method, the object is \"" + sb + "\"");
   }
 
   public static void aMethodOnReference(StringBuffer sb) {
      System.out.println("Inside method, before change, the object is \"" + sb + "\"");
      sb = new StringBuffer("world"); // re-assign the reference to the parameter
      sb.append(" peace");            // the re-assigned parameter
      System.out.println("Inside method, after change, the object is \"" + sb + "\"");
   }
}

In caller, before calling the method, the object is "Hello"
Inside method, before change, the object is "Hello"
Inside method, after change, the object is "world peace"
In caller, after calling the method, the object is "Hello"

Reference-Type Argument - Pass-by-Reference or Pass-by-value?

As the object parameter can be modified inside the method, some people called it pass-by-reference. However, in Java, a copy of reference is passed into the method, hence, Java designers called it pass-by-value.

Passing a Primitive as a One-Element Array?

Primitive-type parameters are passed-by-value. Hence, the method is not able to modify the caller's copy. If you wish to let the method to modify the caller's copy, you might pass the primitive-type parameter as a one-element array, which is not recommended.

Method Overloading vs. Overriding

An overriding method must have the same argument list; while an overloading method must have different argument list. You override a method in the subclass. You typically overload a method in the same class, but you can also overload a method in the subclass.

A overriding method:

1. must have the same parameter list as it original.
2. must have the same return-type or sub-type of its original return-type (since JDK 1.5 - called covariant return-type).
3. cannot have more restrictive access modifier than its original, but can be less restrictive, e.g., you can override a protected method as a public method.
4. cannot throw more exceptions than that declared in its original, but can throw less exceptions. It can throw exceptions that is declared in its original or their sub-types.
5. overriding a private method does not make sense, as private methods are not really inherited by its subclasses.
6. You cannot override a non-static method as static, and vice versa.
7. Technically, a subclass does not override a static method, but merely hides it. Both the superclass' and subclass' versions can still be accessed via the classnames.
8. A final method cannot be overridden. An abstract method must be overridden in an implementation subclass (otherwise, the subclass remains abstract).

A overloading method:

1. must be differentiated by its parameter list. It shall not be differentiated by return-type, exception list or access modifier (which generates compilation error). It could have any return-type, exception list or access modifier, as long as it has a different parameter list than the others.
2. can exist in the original class or its sub-classes.

Frequently-Used Packages in JDK API

JDK API is huge and consists of many packages (refer to JDK API specification). These are the frequently-used packages:

• java.lang (the core JDK package): contains classes that are core to the language, e.g., System, String, Math, Object, Integer, and etc.
• java.util: contains utilities such as Scanner, Random, Date, ArrayList, Vector, Hashtable.
• java.io: contains input and output classes for reading files and I/O streams, such as File.
• java.net: contains networking support, such as Socket and URL.
• java.awt (Abstract Windowing Toolkit): contains classes for implementing a graphical user interface, including classes like Frame, Button, CheckBox.
• java.awt.event: contains event handling classes, such as key-press, mouse-click etc.
• java.swing: Advanced GUI classes, e.g., JFrame, JButton, JApplet, etc.
• java.applet: contains classes for implementing Java applets.
• java.sql: contains classes for database programming, such as Connection, Statement, ResultSet.
• Many others.

Package java.lang Frequently-used Classes

"java.lang" is the Java core language package, which contains classes central to the Java language. It is implicitly "imported" into every Java program. That is, no explicit "import" statement is required for using classes in java.lang.

Frequently-used classes in "java.lang" are:

• String, StringBuffer and StringBuilder: String is immutable whereas StringBuffer/StringBuilder is mutable. StringBuffer is thread-safe; while StringBuilder is not thread-safe and is meant for single-thread operations.
• Math: contains public static final fields PI and E; and many public static methods such as random(), sqrt(), sin(), cos(), asin(), acos(), log(), exp(), floor(), ceil(), pow(), and etc.
• Wrapper class for primitive types: Byte, Integer, Short, Long, Float, Double, Character, and Boolean. The wrapper class is used to wrap a primitive type into a Java class. They are used when a class is needed for purpose such as using multithreading, synchronization and collection. They also contains static utility methods (such as Integer.parseInt()) and static constants (such as Integer.MAX_VALUE).
• System: contains the static variables in, out, and err, corresponds to the standard input, output, and error streams.
• Object: the common root class for all the Java classes. This common root class defines the baseline behaviors needed to support features like multithreading (lock and monitor), synchronization (wait(), notify(), notifyAll()), garbage collection, equals(), hashcode() and toString().

java.lang.String, StringBuilder & StringBuffer

Read "Java String is Special".

Wrapper Classes for Primitive Types

The designers of Java language retain the primitive types in an object-oriented language, instead of making everything object, so as to improve the runtime efficiency and performance. However, in some situations, an object is required instead of a primitive value. For examples,

• The data structures in the Collection framework, such as the "dynamic array" ArrayList and Set, stores only objects (reference types) and not primitive types.
• Object is needed to support synchronization in multithreading.
• Objects are needed, if you wish to modify the arguments passed into a method (because primitive types are passed by value).

JDK provides the so-called wrapper classes that wrap primitive values into objects, for each of the eight primitive types - Byte for byte, Short for short, Integer for int, Long for long, Float for float, Double for double, Character for char, and Boolean for boolean, as shown in the class diagram.

Wrapper Classes are Immutable

Each of the wrapper classes contains a private member variable that holds the primitive value it wraps. The wrapped value cannot be changed. In other words, all the wrapper classes are immutable.

Wrap via Constructors

Each of the wrapper classes has a constructor that takes in the data type it wraps. For examples:

// Wrap an int primitive value into an Integer object
Integer aIntObj = new Integer(5566);
// Wrap a double primitive value into a Double object
Double aDoubleObj = new Double(55.66);
// Wrap a char primitive value into a Character object
Character aCharObj = new Character('z');
// Wrap a boolean primitive value into a Boolean object
Boolean aBooleanObj = new Boolean(true);

All wrapper classes, except Character, also have a constructor that takes a String, and parse the String into the primitive value to be wrapped.

Static factory method valueOf() (JDK 5)

The constructors had been deprecated in JDK 9. You should use static factory method valueOf() to construct an instance.

For examples, the following valueOf() are defined in the Integer class:

// java.lang.Integer
public static Integer valueOf(int i)
public static Integer valueOf(String s)
public static Integer valueOf(String s, int radix)

For example,

public class WrapperClassTest {
   public static void main(String[] args) {
      //Integer iObj1 = new Integer(11);
      //warning: [deprecation] Integer(int) in Integer has been deprecated

      Integer iObj2 = Integer.valueOf(22);
      System.out.println(iObj2);  //22
      Integer iObj3 = Integer.valueOf("33");
      System.out.println(iObj3);  //33
      Integer iObj4 = Integer.valueOf("1ab", 16);  // radix
      System.out.println(iObj4);  //427

      Integer iObj5 = 44;   // auto-box from int to Integer
      int i5 = iObj5;       // auto-unbox from Integer to int
      System.out.println(i5);  //44
   }
}

Unwrap via xxxValue() methods

The abstract superclass Number defines the following xxxValue() methods to unwrap, which are implemented in concrete subclasses Byte, Short, Integer, Long, Float, Double. In other words, you can get an int or double value from an Integer object.

// In classes Byte, Short, Integer, Long, Float and Double
public byte byteValue()              // returns the wrapped "numeric" value as a byte
public short shortValue()            //             ...                     as a short
public abstract int intValue()       //             ...                     as an int
public abstract long longValue()     //             ...                     as a long
public abstract float floatValue()   //             ...                     as a float
public abstract double doubleValue() //             ...                     as a double

Similarly, the Character and Boolean classes have a charValue() and booleanValue(), respectively.

// In Character class
public char charValue()         // Returns as char
// In Boolean class
public boolean booleanValue()   // Returns as boolean

Example

// Wrap a primitive int into an Integer object
Integer intObj = new Integer(556677);
// Unwrap
int i = intObj.intValue();
short s = intObj.shortValue();  // truncate
byte b = intObj.byteValue();    // truncate
   
// Wrap a primitive double into an Double object
Double doubleObj = new Double(55.66);
// Unwrap
double d = doubleObj.doubleValue();
int i1 = doubleObj.intValue();  // truncate
   
// Wrap a primitive char into an Character object
Character charObj = new Character('z');
// Unwrap
char c = charObj.charValue();
   
// Wrap a primitive boolean into a Boolean object
Boolean booleanObj = new Boolean(false);
// Unwrap
boolean b1 = booleanObj.booleanValue();

Constants - MIN_VALUE, MAX_VALUE and SIZE

All wrapper classes (except Boolean) contain the following constants, which give the minimum, maximum, and bit-length.

// All except Boolean
public static final type MIN_VALUE   // Minimum value
public static final type MAX_VALUE   // Maximum value
public static final int SIZE         // Number of bits
// Float and Double only
public static final int MAX_EXPONENT // Maximum exponent 
public static final int MIN_EXPONENT // Maximum exponent

For examples:

// Integer class
System.out.println(Integer.MAX_VALUE);   // 2147483647
System.out.println(Integer.MIN_VALUE);   // -2147483648
System.out.println(Integer.SIZE);        // 32
// Double class
System.out.println(Double.MAX_VALUE);    // 1.7976931348623157E308
System.out.println(Double.MIN_VALUE);    // 4.9E-324
System.out.println(Double.SIZE);         // 64
System.out.println(Double.MAX_EXPONENT); // 1023
System.out.println(Double.MIN_EXPONENT); // -1022

Static Methods for Parsing Strings

Each of the wrapper classes (except Character) also contain a static method to parse a given String into its respective primitive value:

// Byte class
public static byte parseByte(String s) throws NumberFormatException
// Short class
public static short parseShort(String s) throws NumberFormatException
// Integer class
public static int parseInt(String s) throws NumberFormatException
// Long class
public static long parseLong(String s) throws NumberFormatException
// Float class
public static float parseFloat(String s) throws NumberFormatException
// Double class
public static double parseDouble(String s) throws NumberFormatException
// Boolean class
public static boolean parseBoolean(String s)  
    // returns true for string "true" (case insensitive); returns false otherwise

For examples:

// Parse a String into int. Throw NumberFormatException if the String is not valid
int i = Integer.parseInt("5566");
i = Integer.parseInt("abcd");       // Runtime Error: NumberFormatException
i = Integer.parseInt("55.66");      // Runtime Error: NumberFormatException
 
// Parse a String into double
double d = Double.parseDouble("55.66");

Auto-Boxing & Auto-Unboxing (JDK 1.5)

Prior to JDK 1.5, the programmers have to explicitly wrap a primitive value into an object, and explicitly unwrap an object to get a primitive value. For example,

// Pre-JDK 1.5
Integer intObj = new Integer(5566);    // wrap int to Integer
int i = intObj.intValue();             // unwrap Integer to int
 
Double doubleObj = new Double(55.66);  // wrap double to Double
double d = doubleObj.doubleValue();    // unwrap Double to double

The pre-JDK 1.5 approach involves quite a bit of code to do the wrapping and unwrapping. Why not ask the compiler to do the wrapping and unwrapping automatically? JDK 1.5 introduces a new feature called auto-boxing and unboxing, where the compiler could do the wrapping and unwrapping automatically for you based on their contexts. For example:

// Java SE 5.0
Integer intObj = 5566;    // autobox from int to Integer
int i = intObj;           // auto-unbox from Integer to int
 
Double doubleObj = 55.66; // autoboxing from double to Double
double d = doubleObj;     // auto-unbox from Double to double

With the auto-boxing and unboxing, your can practically ignore the distinction between a primitive and its wrapper object.

java.lang.Math - Mathematical Functions & Constants

The java.lang.Math class provides mathematical constants (PI and E) and functions (such as random(), sqrt()). A few functions are listed below for references. Check the JDK API specification for details.

// static constants
public static double Math.PI;             // constant π
public static double Math.E;              // constant e
// static methods
public static double Math.random();       // generate a random number btw 0.0 & 1.0
public static double Math.sin(double x);  // sine function
public static double Math.exp(double x);  // exponential function
public static double Math.log(double x);  // natural logarithm of x
public static double Math.pow(double x, double y);  // x raised to power of y
public static double Math.sqrt(double x); // square root of x

For examples:

double radius = 1.1;
double area = radius * radius * Math.PI;
int number = (int)Math.pow(2, 3);  // int 2 and 3 implicitly promoted to double
                                   // invoke pow(double, double) which return a double
                                   // cast the result back to an int

Take note that Math class is final - you cannot create subclasses. The constructor of Math class is private - you cannot create instances.

java.lang.Object - The Common Java Root Class

java.lang.Object is the superclass of all Java classes. In other words, all classes are subclass of Object - directly or indirectly. A reference of class Object can hold any Java object, because all Java classes are subclasses of Object. In other word, every Java class "is a" Object.

Java adopts a single common root class approach in its design, to ensure that all Java classes have a set of common baseline properties. The Object class defines and implements all these common attributes and behaviors that are necessary of all the Java objects running under the JVM. For example,

• Ability to compare itself to another object, via equals() and hashcode().
• Provides a text string description, via toString().
• Inter-thread communication, via wait(), notify() and notifyAll().
• Automatic garbage collection.

The Object class has the following public methods:

// The following methods must be overridden to be used
public boolean equals(Object obj);
public int hashCode();
// The following methods may be overridden
protected Object clone();
protected void finalize();
public String toString();
// The following methods are final and cannot be overridden
public final Class getClass();
public final void wait(...);
public final void notify();
public final void notifyAll();

• The method getClass() returns a runtime representation of the class in a Class object. A Class object exists for all the objects in Java. It can be used, for example, to discover the fully-qualified name of a class, its members, its immediate superclass, and the interfaces that it implemented. For example,

  objectName.getClass().getName()      // retrieve the class name
  objectName.getClass().newInstance()  // create a new instance

• The method toString() returns a text string description of the object's current state, which is extremely useful for debugging. The toString() is implicitly called by println() and the string concatenation operator '+'. The default implementation in Object returns the classname followed by it hash code (in hexadecimal) (e.g., java.lang.Object@1e78fc6). This method is meant to be overridden in the subclasses.
• The method equals() defines a notion of object equality, based on the object's contents rather than their references. However, the default implementation in Object class use "==" which compares the object's references. This method is meant to be overridden in the subclasses to compare the content via "deep" comparison, rather than references. The equals() shall be reflective and transitive, i.e., a.equals(b) is true, b.equals(a) shall be true; if a.equals(b) and b.equals(c) are true, then a.equals(c) shall be true.
• The method hashCode() maps an object into a hash value. The same object must always produce the same hash value. Two objects which are equals() shall also produce the same hash value.
• The method clone() is used to make a duplicate of an object. It creates an object of the same type from an existing object, and initializes the new object’s member variables to have the same value as the original object. The object to be cloned must implement Cloneable interface. Otherwise, a CloneNotSupportedException will be thrown. For reference type variable, only the reference is cloned, not the actual object.
• The methods wait(), notify(), notifyAll() are used in concurrent (multithread) programming.
• The method finalize() is run before an object is destroyed (i.e., destructor). It can be used for cleanup operation before the object is garbage-collected.

java.lang.System

The System class contains three static variables System.in, System.out and System.err, corresponding to the standard input, output and error streams, respectively.

• System.exit(returnCode): terminate the program with the return code.
• System.currentTimeMillis() & System.nanoTime(): get the current time in milliseconds and nanoseconds. These methods can be used for accurate timing control.
• System.getProperties(): retrieving all the system properties.

java.lang.Runtime

Every Java program is associated with an instance of Runtime, which can be obtained via the static method Runtime.getRuntime(). You can interface with the operating environment via this Runtime, e.g., exec(String command) launches the command in a separate process.

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// Call up another program
import java.io.IOException;
   
public class ExecTest {
   public static void main(String[] args) {
      try {
         Runtime.getRuntime().exec("calc.exe");
      } catch (java.io.IOException ex) {
         ex.printStackTrace();
      }
   }
}

Package java.util Frequently-Used Classes

java.util.Random

Although Math.random() method can be used to generate a random double between [0.0, 1.0), the java.util.Random class provides more extensive operations on random number, e.g., you can set a random number generator with a initial seed value, to generate the same sequence of random values repeatedly.

Example

import java.util.Random;
public class TestRandomClass {
   public static void main(String[] args) {
      // Allocate a pseudo-random number generator with default random seed
      Random random = new Random();
         
      // Generate the next 10 pseudo-random uniformly distributed int value 
      //   between 0(inclusive) and 100(exclusive) 
      for (int i = 0; i < 10; ++i) { 
         System.out.print(random.nextInt(100) + " ");
      }
      System.out.println();
      
      // Generate the next pseudo-random uniformly distributed double/float value 
      //   between 0.0(inclusive) and 1.0(exclusive) 
      System.out.println(random.nextDouble());
      System.out.println(random.nextFloat());
      
      // Allocate a pseudo-random number generator with the specified seed value
      Random anotherRandom = new Random(12345);
      // Generate the "same" sequence of 10 integers
      for (int i = 0; i < 10; ++i) { 
         System.out.print(anotherRandom.nextInt(100) + " ");
      }
      System.out.println();
   }
}

Example: Simulating throw of 3 dice.

/**
 * Throw 3 dices and get the total score. Also examine for 
 * - 3-of-a-kind (all 3 dice are the same);
 * - pair (any two dice are the same);
 * - special (one dice is more than the sum of the other two)
 */
import java.util.Random;
public class DiceSimulation {
   public static void main(String[] args) {
      Random random = new Random();   // Allocate a random generator
      int[] diceScores = new int[3];  // Allocate 3 dice
      int totalScore = 0;
      
      // Throw the dice
      for (int i = 0; i < diceScores.length; ++i) {
         diceScores[i] = random.nextInt(6) + 1;  // 1 to 6
      }
      
      // Compute total score
      System.out.print("The dice are:");
      for (int diceScore : diceScores) {
         totalScore += diceScore;
         System.out.print(" " + diceScore);
      }
      System.out.println();
      System.out.println("The total score is " + totalScore);
      
      // Check for 3-of-a-kind and pair
      if (diceScores[0] == diceScores[1]) {
         if (diceScores[0] == diceScores[2]) {
            System.out.println("It's a 3-of-a-kind");
         } else {
            System.out.println("It's a pair");
         }
      } else {
         if (diceScores[0] == diceScores[2] || diceScores[1] == diceScores[2]) {
            System.out.println("It's a pair");
         }
      }
      
      // Check for special
      if ((diceScores[0] > diceScores[1] + diceScores[2]) ||
          (diceScores[1] > diceScores[0] + diceScores[2]) ||
          (diceScores[2] > diceScores[0] + diceScores[1])) {
         System.out.println("It's a special");
      }
   }
}

java.util.Scanner & java.util.Formatter (JDK 1.5)

Read "Formatted-text I/O".

java.util.Arrays

The Arrays class contains various static methods for manipulating arrays, such as comparison, sorting and searching.

For examples,

• The static method boolean Arrays.equals(int[] a, int[] b), compare the contents of two int arrays and return boolean true or false.
• The static method void Arrays.sort(int[] a) sorts the given array in ascending numerical order.
• The static method int binarySearch(int[] a, int key) searches the given array for the specified value using the binary search algorithm.
• others

Example: See "Collection Framework".

java.util.Collection

See "Collection Framework".

Package java.text Frequently-Used Classes

The java.text package contains classes and interfaces for handling text, dates, numbers and currencies with locale (internationalization) support.

[TODO] compare with (JDK 1.5) String.format() and format specifiers and Formatter/Scanner - check for locale support.

The NumberFormat/DecimalFormat and DateFormat/SimpleDateFormat supports both output formatting (number/date -> string) and input parsing (string -> number/date) in a locale-sensitive manner for internationalization (i18n).

java.text.NumberFormat

The NumberFormat class can be used to format numbers and currencies for any locale. To format a number for the current Locale, use one of the static factory methods:

String myString = NumberFormat.getInstance().format(myNumber);

The available factory methods are:

public static final NumberFormat getInstance();                // Returns a general-purpose number format
public static final NumberFormat getInstance(Locale l);
public static final NumberFormat getNumberInstance();          // Returns a general-purpose number format
public static final NumberFormat getNumberInstance(Locale l); 
public static final NumberFormat getIntegerInstance();         // Returns a integer number format
public static final NumberFormat getIntegerInstance(Locale l);
public static final NumberFormat getCurrencyInstance();        // Returns a currency number format
public static final NumberFormat getCurrencyInstance(Locale l);
public static final NumberFormat getPercentInstance();         // Returns a percent number format
public static final NumberFormat getPercentInstance(Locale l);

The default currency format rounds the number to two decimal places; the default percent format rounds to the nearest integral percent; the default integer format rounds to the nearest integer.

Example 1

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import java.text.NumberFormat;
import java.util.Locale;
 
public class TestNumberCurrencyFormat {
   public static void main(String[] args) {
      Locale[] locales = { Locale.US, Locale.FRANCE, Locale.JAPAN };
 
      for (Locale loc:locales) {
         NumberFormat formatter = NumberFormat.getInstance(loc);
         String formattedNumber = formatter.format(123456789.12345);
         System.out.format("%15s: %s\n", loc.getDisplayCountry(), formattedNumber);
      }
 
      for (Locale loc:locales) {
         NumberFormat formatter = NumberFormat.getCurrencyInstance(loc);
         String formattedNumber = formatter.format(123456789.12345);
         System.out.format("%15s: %s\n", loc.getDisplayCountry(), formattedNumber);
      }
 
   }
}

  United States: 123,456,789.123
         France: 123 456 789,123
          Japan: 123,456,789.123
  United States: $123,456,789.12
         France: 123 456 789,12 €
          Japan: ￥123,456,789

Example 2

In this example, we use static method NumberFormat.getAvailableLocales() to retrieve all supported locales, and try out getInstance(), getIntegerInstance(), getCurrencyInstance(), getPercentInstance().

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import java.util.Locale;
import java.text.NumberFormat;
 
public class NumberFormatTest {
   public static void main(String[] args) {
      // Print a number using the localized number, integer, currency,
      //  and percent format for each available locale
      Locale[] locales = NumberFormat.getAvailableLocales();
      double myNumber = -1234.56;
      NumberFormat format;
 
      // General Number format
      System.out.println("General Format:");
      for (Locale locale : locales) {
         if (locale.getCountry().length() == 0) continue;  // Skip language-only locales
         format = NumberFormat.getInstance(locale);
         System.out.printf("%40s -> %s%n", locale.getDisplayName(), format.format(myNumber));
      }
 
      // Integer format
      System.out.println("Integer Format:");
      for (Locale locale : locales) {
         if (locale.getCountry().length() == 0) continue;  // Skip language-only locales
         format = NumberFormat.getIntegerInstance(locale);
         System.out.printf("%40s -> %s%n", locale.getDisplayName(), format.format(myNumber));
      }
 
      // Currency format
      System.out.println("Currency Format:");
      for (Locale locale : locales) {
         if (locale.getCountry().length() == 0) continue;  // Skip language-only locales
         format = NumberFormat.getCurrencyInstance(locale);
         System.out.printf("%40s -> %s%n", locale.getDisplayName(), format.format(myNumber));
      }
 
      // Percent format
      System.out.println("Percent Format:");
      for (Locale locale : locales) {
         if (locale.getCountry().length() == 0) continue;  // Skip language-only locales
         format = NumberFormat.getPercentInstance(locale);
         System.out.printf("%40s -> %s%n", locale.getDisplayName(), format.format(myNumber));
      }
   }
}

You can also use the NumberFormat to parse an input string (represent in the locale) to a Number:

public Number parse(String source) throws ParseException

java.text.DecimalFormat

The DecimalFormat class is a subclass of NumberFormat, which adds support for formatting floating-point numbers, such as specifying precision, leading and trailing zeros, and prefixes and suffixes. A DecimalFormat object has a pattern to represent the format of the decimal number, e.g., "#,###,##0.00", where 0 denotes zero padding, and # without the zero-padding.

To use a DecimalFormat with the default locale, invoke its constructor with the pattern, e.g.,

double d = -12345.789
DecimalFormat format = new DecimalFormat("$#,###,##0.00"); // default locale
System.out.println(format.format(d));                      // -$12,345.79
 
format.applyPattern("#,#00.0#;(#,#00.0#)"); // "positive;negative"
System.out.println(format.format(d));       // (12,345.79)

To use a DecimalFormat with locale, get a NumberFormat by calling the getInstance() and downcast it to DecimalFormat. For example,

double d = -12345.789;
NumberFormat nf = NumberFormat.getInstance(Locale.GERMAN);
if(nf instanceof DecimalFormat) {
   DecimalFormat df = (DecimalFormat) nf;
   df.applyPattern("##,#00.00#");
   System.out.println(df.format(d));  // -12.345,789
}

java.text.DateFormat

The DateFormat class can be used to format a date instance with locale.

Read "Date and Time".

To format a date/time for the current locale, use one of the static factory methods:

myString = DateFormat.getDateInstance().format(myDate);

The available factory methods for getting a DateFormat instance are:

public static final DateFormat getTimeInstance();
public static final DateFormat getTimeInstance(int timeStyle);   // DateFormat.FULL, LONG, MEDIUM and SHORT
public static final DateFormat getTimeInstance(int timeStyle, Locale l)
public static final DateFormat getDateInstance();
public static final DateFormat getDateInstance(int dateStyle);
public static final DateFormat getDateInstance(int dateStyle, Locale l)
public static final DateFormat getDateTimeInstance();
public static final DateFormat getDateTimeInstance(int dateStyle, int timeStyle);
public static final DateFormat getDateTimeInstance(int dateStyle, int timeStyle, Locale l);
public static final DateFormat getInstance();    // default DateTime formatter in SHORT style

The exact display for each style depends on the locales, but in general,

• DateFormat.SHORT is completely numeric, such as 12.13.52 or 3:30pm
• DateFormat.MEDIUM is longer, such as Jan 12, 1952
• DateFormat.LONG is longer, such as January 12, 1952 or 3:30:32pm
• DateFormat.FULL is pretty completely specified, such as Tuesday, April 12, 1952 AD or 3:30:42pm PST.

You can also use the DateFormat to parse an input string containing a date in the locale to a Date object.

public Date parse(String source) throws ParseException

java.text.SimpleDateFormat

The SimpleDateFormat is a concrete subclass of DataFormat for formatting and parsing dates in a locale-sensitive manner. It supports output formatting (date to string), input parsing (string to date), and normalization.

You can construct a SimpleDateFormat via one of its constructors:

public SimpleDateFormat(String pattern);
public SimpleDateFormat(String pattern, Locale locale);

For example, [TODO]

Writing Javadoc

A great feature in Java is the documentation can be integrated with the source codes, via the so-called JavaDoc (Java Documentation) comments. In other languages, documentation typically is written in another file, which easily gets out-of-sync with the source codes.

JavaDoc comments begin with /** and end with */. They are meant for providing API documentation to the users of the class. JavaDoc comments should be provided to describe the class itself; and the public variables, constructors, and methods.

You can use JDK utility javadoc to extract these comments automatically and produce API documentation in a standard format.

With JavaDoc comments, you can keep the program documentation inside the same source file instead of using another documentation file. This provides ease in synchronization.

JavaDoc comments and API documentation are important for others to re-use your program. Write JavaDoc comments while you are writing the program. Do not leave them as after-thought.

Example

Let's add the JavaDoc comments to all the public entities of our Circle class.

/**
 * The Circle class models a circle with a radius and color.
 *
 * @author CHC
 */
public class Circle {

   // private instance variables
   private double radius;
   private String color;

   /**
    * Construct a circle with default radius of 1.0 and color of blue.
    */
   public Circle() {
      radius = 1.0;
      color = "blue";
   }

   /** Construct a circle with the given radius and color.
    *  @param radius  The radius of the circle
    *  @param color   The color of the circle
    */
   public Circle(double radius, String color) {
      this.radius = radius;
      this.color = color;
   }

   /** Return the radius of the circle.
    *  @return  The radius of the circle.
    */
   public double getRadius() {
      return radius;
   }

   /** Set the radius of the circle.
    *  @param radius  The radius of the circle to be set.
    */
   public void setRadius(double radius) {
      this.radius = radius;
   }

   /** Return the color of the circle.
    *   @return  The color of the circle.
    */
   public String getColor() {
      return color;
   }

   /** Set the color of the circle.
    *   @param color  The color of the circle to be set.
    */
   public void setColor(String color) {
      this.color = color;
   }

   /** Return the area of the circle.
    *  @return  The area of the circle.
    */
   public double getArea() {
      return radius * radius * Math.PI;
   }

   /** Return a short description of this instance.
    *  @return  A short string description.
    */
   public String toString() {
      return "Circle[radius=" + radius + ", color=" + color + "]";
   }
}

You can produce the standard API documentation, via JDK utility javadoc, as follows:

// cd to the source directory
javadoc Circle.java

Browse the resultant API, by opening the "index.html".

Miscellaneous

Are private members inherited by the subclasses?

NO.

Study the following example:

class A {
   private void foo() {  // private members are accessible within this class only
      System.out.println("Class A runs private method foo()");
   }
}

class B extends A {
   //@Override
   //compilation error: method does not override or implement a method from a supertype
   public void foo() {
      System.out.println("Class B runs public method foo()");
      //super.foo();  //compilation error: foo() has private access in A
   }
}

public class PrivateInheritanceTest {
   public static void main(String[] args) {
      A a1 = new A();
      //a1.foo();  //compilation error: foo() has private access in A

      B b1 = new B();
      b1.foo();    //Class B runs public method foo()

      A a2 = new B();
      //a2.foo();  //compilation error: foo() has private access in A
                   //no polymorphism because foo() is private in A
   }
}

static methods are inherited but cannot be overridden in subclasses

References:

1. Overriding and Hiding Methods @ https://docs.oracle.com/javase/tutorial/java/IandI/override.html.

Study the following examples,

class A {
   public static void foo() {
      System.out.println("A runs foo()");
   }
   public static void bar() {
      System.out.println("A runs bar()");
   }
}

// This subclass inherits both static methods foo() and bar()
class B extends A {
   //@Override  //compilation error: static methods cannot be annotated with @Override
   public static void foo() {
      System.out.println("B runs foo()");
   }
   //public void bar() {}  // non-static
   //compilation error: bar() in B cannot override bar() in A
   
   public void hello() {  // non-static instance
      foo();
      bar();
   }
   public static void helloStatic() {
      foo();
      bar();
   }
}

public class StaticInheritanceTest {
   public static void main(String[] args) {
      A a = new A();
      a.foo();  //A runs foo()
      A.foo();  //A runs foo()
      a.bar();  //A runs bar()
      A.bar();  //A runs bar()

      B b = new B();
      b.foo();  //B runs foo() (hide the superclass version)
      B.foo();  //B runs foo()
      b.bar();  //A runs bar() (inherited from superclass)
      B.bar();  //A runs bar()
      b.hello();  //B runs foo()
                  //A runs bar() (inherited from superclass)
      b.helloStatic();  //B runs foo()
                        //A runs bar()
      B.helloStatic();  //B runs foo()
                        //A runs bar()

      // Test Polymorphism
      A a1 = new B();
      a1.foo();  //A runs bar() (non-polymorphic!!!)
      a1.bar();  //A runs bar()
   }
}

In summary:

• static methods of superclass are inherited by the subclasses. You can invoke the inherited method from the subclasses, or subclasses' instances.
• static methods (unlike instance methods) CANNOT be overridden in the subclasses (you CANNOT annotate with @Override). However, a subclass can provide its own version of the inherited static method, which HIDE the inherited version. HIDDEN methods do NOT take part in polymorphism!!!
• You cannot use a non-static method to hide a static method, which triggers "compilation error: cannot override".

Latest version tested: JDK 1.13.1
Last modified: February, 2020