Inteface

Question

1. If I have a set of methods that should be provided by certain classes, but I don't know how to implement these methods now. What should I do?.
   For example, I want to create a computer program that works like a music player (like MP3 player, or CD player). I know that they all should have the following methods.

   public void play();
   public void stop();
   public void pause();
   public void fastForward();
   public void rewind();
   public void changeVolTo(int level);
   

   My purpose is to create a superclass (let us call in BasicMusicPlayer.java) and let all other music players extend it. But I don't know how (and impossible) to implement them in this class What can I do?
2. How can I create a class that extends two or more classes?

Answer

1. You know all the method names. But you don't know how to implement them. You can create a class that provides the method declarations only, without providing the methods' body.
   This is very similar to header files in C++.
   Instead of use public class BasicMusicPlayer, we use the following code

   // save in BasicMusicPlayer.java
   public interface BasicMusicPlayer { 
       void play();               
       void stop();                  
       void pause();                
       void fastForward();            
       void rewind();                
       void changeVolTo(int level); 
   }
   

   Notice all the methods in an interface are public. So we can skip the word public in the body.
   The way to use it is

   public class MP3Player implements BasicMusicPlayer {
       public void play(){
          // your implementation
       }               
       public void stop(){
          // your implementation       
       }            
       public void pause(){
          // your implementation
       }               
       public void fastForward(){
          // your implementation
       }            
       public void rewind() {
          // your implementation
       }              
       public void changeVolTo(int level){
          // your implementation    
       }
       
       // constructors and other methods here
   }
   

   You have to override all methods provided in the inteface.
2. It is impossible to extends 2 or more classes at the same time. You can do it in C++ but you can't do it in java.
   However, we can implements (the meaning is similar to extends) one or more interface (the meaning is similar to class).
   Example we can have

   public class MP3Player implements BasicMusicPlayer 
   public class CDPlayer extends JFrame implements BasicMusicPlayer
   public class RecordablePlayer extends JFrame implements BasicMusicPlayer, Recordable
   

   here Recordable is another interface.The list of interfaces are seperated by comma.

Here are the correspondings :

interface  <--------------> class
extends    <--------------> implements

Here are the differences :
• You can only extend one class but you can implement more then one interface
• You have to override all the methods provided by the interface.

Using an Interface as a Type

public double getPrice(BasicMusicPlayer player) 

Summary

• The way to declare it is
  
• The interface body consist of
  • method declaration : with method name, argument list, and return type. All the methods are public by default, so you can skip the word public. And you end every method declaration with ";".
  • constants : you can declare public static final member for class member. Here is an example:

       public interface BasicMusicPlayer { 
          final int MAX_LEVEL = 10; 
          void play();               
          void stop();                  
          void pause();                
          void fastForward();            
          void rewind();                
          void changeVolTo(int level); 
      }
      

  • Actually most of the Swing classes implements javax.swing.SwingConstants. The class provide most of the constants Swing classes use. eg. NORTH, SOUTH, TOP, BOTTOM, LEFT, RIGHT ...