下面ChangeTitle()中对b1和b2按钮构造监听器，实现当单击b1按钮时标题变为students，当单击b2按钮时标题变为teachen。请将程序补充完整。
public ChangeTitle()
super("Title Bar");
b1. addActionListener(this); 【15】
Jpanel pane = new Jpanel();
Pane.add(b1);
Pane.add(b2);
SetContentPane(pane);public void actionPefformed(ActionEvent evt)
Object sourve = evt. getSource();
if (sourve ==b1)
setTitle("Students");
else if (source == b2)
setTitle("Teachers ");
repaint();