填空题

本题的功能是定义图形按钮。窗口中有两个图形按钮，当鼠标移上去时，图形按钮的图形将改变，用鼠标单击按钮时图形按钮的图形又发生改变，当鼠标左键松开或者移开鼠标后，图形按钮的图形又恢复原样。
import javax. swing. * ;
import java. awt. * ;
public class java3 extend JButton
public java3(Icon icon, Icon pressed, Icon rollover)
this(icon);
setFocusPainted (alse);
setRolloverEnabled (true);
setRolloverIcon(rollover);
setPressedIcon(pressed);
setBorderPainted (false);
setContentAreaFilled (false);

public static void main(String[] args)
java3 b1=new java3(
new Imagelcon("java3-redcube. gif"),
new ImageIcon("java3-redpaw. gif"),
new Imagelcon("java3-reddiamond. gif"));
java3 b2=new java3(
new ImageIcon("java3-bluecube. gif"),
new Imagelcon("java3-bluepaw. gif"),
new Imagelcon("java3-bluediamond. gif"));
JFrame f=new JFrame();
f. setTitle("java3");
f. setDefaultCloseOperation(JFrame. EXIT_ON_CLOSE);
c=f. getContentpane();
c. setLayout(new FlowLayout());
c. add(b1);
c. add(b2);
f. pack();
f. setVisible(true);