关于接口为内部时和为外部时的区别与联系

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首先说明，无论接口是在类内部还是外部，都需要覆盖接口定义的所有方法。

至于原因就去查看java教材关于接口的那一章讲解。

一、当接口为内部时，在类的头部即public class YY 后不必写implements XX-interface

如：

public class J_Panel extends JPanel{ public J_Panel() {  addMouseListener(new MouseListener()  {   public void mouseExited(MouseEvent e)   {}   public void mouseClicked(MouseEvent e)   {}   public void mouseEntered(MouseEvent e)   {}   public void mousePressed(MouseEvent e)   {}   public void mouseReleased(MouseEvent e)   {}  }  ); // end of addMouseListener   addMouseMotionListener(new MouseMotionListener()  {   public void mouseMoved(MouseEvent e)   {}   public void mouseDragged(MouseEvent e)   {}  }  ); // End of addMouseMotionListener } // End of the conductor of J_Panel protected void paintComponent(Graphics g) {  } // End of the function of paintComponent}

注：若写上了implement MouseListener，

则编译时会发生错误:

class J_Panel不是抽象的，未覆盖java.awt.event.MouseListener中的抽象方法.....

就是说，内部接口的实现并不能覆盖implements MouseListener的抽象方法。

二、而当接口为外部时，必须在类的头部即public class YY 后写上implements XX-interface

如：

public class ScribblePanel extends JPanel implements MouseListener, MouseMotionListener{  public ScribblePanel() {  // Register listener for the mouse event  addMouseListener(this);  addMouseMotionListener(this); }// interface MouseListener public void mouseClicked(MouseEvent e) {} public void mouseEntered(MouseEvent e) {} public void mouseExited(MouseEvent e) {} public void mouseReleased(MouseEvent e) {} public void mousePressed(MouseEvent e) {} // interface MouseMotionListener public void mouseDragged(MouseEvent e) {} public void mouseMoved(MouseEvent e) {}}

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