下面ChangeTitle()中对b1和b2按键构造监听器，实现当按下b1键时标题变为Students，当按下b2键时标题变为Teachers。添加空缺的语句。
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public ChangeTitle()
super("Title Bar");
b1.addActionListener(this);
【11】 ;
Jpanel pane = new Jpane1();
Pane.add(b1);
Pane.add(b2);
SetContentPane(pane);
public void actionPerformed(ActionEvent evt)
Object sourve = evt.getSource();
if (sourve ==b1)
setTitle("Students");
else if (source == b2)
setTitle("Teachers");
repaint();
...
