Introduction Terminology Expressions Control Objects Inheritance Technology Summary

Objects

An object is an instance of a class. In Java, a program is executed by calling the static main method a class. Look for example at the HelloWorld class in slide Java-objects-1.

Hello World -- Java (1)


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

slide: Java -- objects (1)

The HelloWorld example is taken from a collection of hello world programs located at www.latech.edu/~acm/HelloWorld.shtml . See hello for a sample of this collection.

To illustrate the use of interface definitions, slide Java-objects-2 presents a slightly modified version. The actual HelloWorld class announces that it implements the World interface.

Hello World - interface


  public interface World {
  public void hello();
  };

Hello World - class


  public class HelloWorld implements World {
  public void hello() {
         System.out.println("Hello World");
         }
  };

slide: Java -- objects (2)

Both classes make use of the standard out stream defined in the class System to emit their message to the world.