关于接口为内部时和为外部时的区别与联系

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)
{}
}