某小应用程序有一个由3个单选按钮组成的颜色选择组，当选中某种颜色时，窗口的背景颜色将作相应的变化。这里给出的是窗口子类My Window的定义的框架以及其构造函数的定义。class MyWindow extends JFrame implements ________________

{…

MyWindow(String s){

super(s);

Container con = this.getContentPane();

con.setLayout(new GridLayout(3,2));

this.setLocation(100, 100);

this.setSize(400, 400);

panel 1 = new Panel l ();

label l = new JLabel("改变前景颜色",JLabel.CENTER);

con.add(label 1 );

con. ________________;

panel 1 .box 1 .addltemListener(this);

panel 1 .box 2.addItemListener(this);

panel 1 .box 3.addItemListener(this);

this. setVisible (true);

this.pack();

}

…

}