Examples

Hello world

The traditional Hello world program can be written in Java as:

// Outputs "Hello, world!" and then exits
public class HelloWorld {
 public static void main(String[] args) {
     System.out.println("Hello, world!");
 }
}

Source files must be named after the public class they contain, appending the suffix .java, for example, HelloWorld.java. It must first be compiled into bytecode, using a Java compiler, producing a file named HelloWorld.class. Only then can it be executed, or 'launched'. The java source file may only contain one public class but can contain multiple classes with less than public access and any number of public inner classes.

A class that is not declared public may be stored in any .java file. 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 class hierarchy is related to the name of the directory in which the .java file is.

The keyword static in front of a method indicates a static method, which is associated only with the class and not with any specific instance of that class. Only static methods can be invoked without a reference to an object. Static methods cannot access any method variables that are not static.

The keyword void indicates that the main method does not return any value to the caller. If a Java program is to exit with an error code, it must call System.exit() explicitly.

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 JavaBean do not use or need a main() method. A java program may contain multiple classes that have main methods, which means that the VM needs to be explicitly told which class to launch from.

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 or as an attribute in a JAR) 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.

Printing is part of a 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 many methods for printing data to standard out, including println(String) which also appends a new line to the passed string.

The string "Hello, world!" is automatically converted to a String object by the compiler.

A more comprehensive example

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

public class OddEven {
  // "input" is the number that the user gives to the computer
  private int input; // a whole number("int" means integer)

  /*
   * This is the constructor method. It gets called when an object of the OddEven type
   * is being created.
   */
  public OddEven() {
  //Code not shown
  }

  // This is the main method. It gets called when this class is run through a Java interpreter.
  public static void main(String[] args) {
      /*
       * This line of code creates a new instance of this class called "number" (also known as an
       * Object) and initializes it by calling the constructor.  The next line of code calls
       * the "showDialog()" method, which brings up a prompt to ask you for a number
       */
      OddEven number = new OddEven();
      number.showDialog();
  }

  public void showDialog() {
      /*
       * "try" makes sure nothing goes wrong. If something does,
       * the interpreter skips to "catch" to see what it should do.
       */
      try {
          /*
           * The code below brings up a JOptionPane, which is a dialog box
           * The String returned by the "showInputDialog()" method is converted into
           * an integer, making the program treat it as a number instead of a word.
           * After that, this method calls a second method, calculate() that will
           * display either "Even" or "Odd."
           */
          input = new Integer(JOptionPane.showInputDialog("Please Enter A Number"));
          calculate();
      } catch (NumberFormatException e) {
          /*
           * Getting in the catch block means that there was a problem with the format of
           * the number. Probably some letters were typed in instead of a number.
           */
          System.err.println("ERROR: Invalid input. Please type in a numerical value.");
      }
  }

  /*
   * When this gets called, it sends a message to the interpreter.
   * The interpreter usually shows it on the command prompt (For Windows users)
   * or the terminal (For Linux users).(Assuming it's open)
   */
  private void calculate() {
      if (input % 2 == 0) {
          System.out.println("Even");
      } else {
          System.out.println("Odd");
      }
  }
}

• 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.
• 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.showDialog(); 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.
• input = new Integer(JOptionPane.showInputDialog("Please Enter A Number")); is a statement that converts the type of String to the primitive data type int by taking advantage of the primitive wrapper class Integer.