1. A simple program most programmers have seen. It just prints "HelloWorld!" to a console.
   

   • public class HelloWorld0 {
       public static void main(String[] args) {
         System.out.println("0 Hello World!");  // output message
       }
     }

   • Source code (example0.zip)
   • Execution result:
     To run the example, execute the "examples0.bat" file that is included with the source code.
2. For a simple program, the above program may be OK but as the application becomes more complex, it may be necessary to change the message.
   If the message changes often, it may become benefitial to create an interface and have the client program choose which implementation of the interface to use. That is, which message to output.
   
   • 
     Greeting interface just contains a greet() method which will be invoked from the application.
     (Greeting.java)

     public interface Greeting {
     	public void greet();                    // method to output message
     }

     GreetingImpl is one implementation of Greeting. To differentiate it from other implementation of Greeting, we'll output "1 Hello World!".
     (GreetingImpl.java)

     public class GreetingImpl implements Greeting {
       public String greet() {
         return "2 Hello World!";
       }
     }}

     Greeting2Impl is other implementation of Greeting. In this implementation, we'll output "1b Hello World!" to distinguish it.
     (Greeting2Impl.java)

     public class Greeting2Impl implements Greeting {
     	public void greet() {
     		System.out.println("1b Hello World!");  // output message
     	}
     }

     Finally, an application to call the implementations. We'll create both implementation. Note that both greeting and greeting2 are of type Greeting even though their implementation differ. Using the Greeting interface allows us to use variable "greeting" for both GreetingImpl and Greeting2Impl.
     (HelloWorld1.java)

     public class HelloWorld1 {
       public static void main(String[] args) {
         Greeting greeting = new GreetingImpl();  // use GreetingImpl
         greeting.greet();                        // output message
     
         greeting = new Greeting2Impl();          // use Greeting2Impl
         greeting.greet();                        // output message
       }
     }

   • Source code (example1.zip)
   • Execution result:
     To run the example, execute the "examples1.bat" file that is included with the source code. Note that "greeting" was first set to "GreetingImpl" to output "1 Hello World!" and then changed to "Greeting2Impl" to output "1b Hello World!".
3. We were able to change the message but what if we also want to change where the message is outputted? Let's try to change the above example so we'll also be able to change the output to System.err as well as System.out.
   
   • We'll make the output routine into an interface - GreetingClient.
     (GreetingClient.java)

     public interface GreetingClient {
       void setGreeting(Greeting greeting);  // set message to output
       void execute();                       // output message
     }

     First, an implementation to output to System.out:
     (GreetingClientImpl.java)

     public class GreetingClientImpl implements GreetingClient {
         private Greeting greeting;
     
         public void setGreeting(Greeting greeting) {  // set message to output
             this.greeting = greeting;
         }
         
         public void execute() {                   // output message to System.out
             System.out.println(greeting.greet());
         }
     }

     And an implementation to output to System.err.
     (GreetingClient2Impl.java)

     public class GreetingClient2Impl implements GreetingClient {
     
         private Greeting greeting;
     
         public void setGreeting(Greeting greeting) {  // set message to output
             this.greeting = greeting;
         }
         
         public void execute() {                    // output message to System.err
             System.err.println(greeting.greet());
         }
     }

     (HelloWorld2.java)

     public class HelloWorld2 {
     	public static void main(String[] args) {
     		Greeting greeting = new GreetingImpl();  // set message to output
                                                  // set how to output message
     		GreetingClient greetingClient = new GreetingClientImpl();
     		greetingClient.setGreeting(greeting);    // set message to output
     		greetingClient.execute();                // output message
     
     		greeting = new Greeting2Impl();          // set message to output
                                                  // set how to output message
     		greetingClient = new GreetingClient2Impl();
     		greetingClient.setGreeting(greeting);    // set message to output
     		greetingClient.execute();                // output message
     	}
     }

   • Source code (example2.zip)
   • Execution result:
     To run the example, execute the "examples2.bat" file that is included with the source code. When the program is executed from the console, there doesn't seem to be any difference between the two implementations to output the messages, but when it is executed from Eclipse, they will be displayed in different color.
     

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