Java index

The syntax of Java is largely derived from C++. However, unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built exclusively as an object oriented language. As a result, almost everything is an object and all code is written inside a class. The exceptions are the intrinsic data types (ordinal and real numbers, boolean values, and characters), which are not classes for performance reasons.

This is a minimal Hello world program in Java:

// Hello.java public class Hello {

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

}

To execute a Java program, the code is saved as a file named Hello.java. It must first be compiled into bytecode using a Java compiler, which produces a file named Hello.class. This class is then launched.

The above example merits a bit of explanation.

   * All executable statements in Java are written inside a class, including stand-alone programs.
   * Source files are by convention named the same as the class they contain, appending the mandatory suffix .java. A class which is declared public is required to follow this convention. (In this case, the class Hello is public, therefore the source must be stored in a file called Hello.java).
   * The compiler will generate a class file for each class defined in the source file. The name of the class file is the name of the class, with .class appended. For class file generation, anonymous classes are treated as if their name was the concatenation of the name of their enclosing class, a $, and an integer.
   * The keyword public denotes that a method can be called from code in other classes, or that a class may be used by classes outside the class hierarchy.
   * The keyword static indicates that the method is a static method, associated with the class rather than object instances.
   * The keyword void indicates that the main method does not return any value to the caller.
   * The method name "main" is not a keyword in the Java language. It is simply the name of the method the Java launcher calls to pass control to the program. Java classes that run in managed environments such as applets and Enterprise Java Beans do not use or need a main() method.
   * The main method must accept an array of String objects. By convention, it is referenced as args although any other legal identifier name can be used. Since Java 5, the main method can also use variable arguments, in the form of public static void main(String... args), allowing the main method to be invoked with an arbitrary number of String arguments. The effect of this alternate declaration is semantically identical (the args parameter is still an array of String objects), but allows an alternate syntax for creating and passing the array.
   * The Java launcher launches Java by loading a given class (specified on the command line) and starting its public static void main(String[]) method. Stand-alone programs must declare this method explicitly. The String[] args parameter is an array of String objects containing any arguments passed to the class. The parameters to main are often passed by means of a command line.
   * The printing facility is part of the Java standard library: The System class defines a public static field called out. The out object is an instance of the PrintStream class and provides the method println(String) for displaying data to the screen while creating a new line (standard out).

An example that better demonstrates object-oriented programming:

// OddEven.java import javax.swing.JOptionPane;

public class OddEven {

   private int input;

   public OddEven() {
       input = Integer.parseInt(JOptionPane.showInputDialog("Please Enter A Number"));
   }

   public void calculate() {
       if (input % 2 == 0)
           System.out.println("Even");
       else
           System.out.println("Odd");
   }

   public static void main(String[] args) {
       OddEven number = new OddEven();
       number.calculate();
   }

}

   * The import statement imports the JOptionPane class from the javax.swing package.
   * The OddEven class declares a single private field of type int named input. Every instance of the OddEven class has its own copy of the input field. The private declaration means that no other class can access (read or write) the input field.
   * OddEven() is a public constructor. Constructors have the same name as the enclosing class they are declared in, and unlike a method, have no return type. A constructor is used to initialize an object that is a newly created instance of the class. In this case, the constructor initializes the input field to the value entered into a JOptionPane input dialog. The dialog returns a String which is converted to an int by the Integer.parseInt(String) method.
   * The calculate() method is declared without the static keyword. This means that the method is invoked using a specific instance of the OddEven class. (The reference used to invoke the method is passed as an undeclared parameter of type OddEven named this.) The method tests the expression input % 2 == 0 using the if keyword to see if the remainder of dividing the input field belonging to the instance of the class by two is zero. If this expression is true, then it prints Even; if this expression is false it prints Odd. (The input field can be equivalently accessed as this.input, which explicitly uses the undeclared this parameter.)
   * OddEven number = new OddEven(); declares a local object reference variable in the main method named number. This variable can hold a reference to an object of type OddEven. The declaration initializes number by first creating an instance of the OddEven class, using the new keyword and the OddEven() constructor, and then assigning this instance to the variable.
   * The statement number.calculate(); calls the calculate method. The instance of OddEven object referenced by the number local variable is used to invoke the method and passed as the undeclared this parameter to the calculate method.
   * For simplicity, error handling has been ignored in this example. Entering a value that is not a number will cause the program to crash. This can be avoided by catching and handling the NumberFormatException thrown by Integer.parseInt(String).