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Question

作如下的修改，我已经测试过了。
        menuItem1.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e)
            {
                cp.removeAll();
                cp.add(p1);
                    cp.validate();
                    cp.repaint();//增加repaint()方法既可。
            }
        });
        menuItem2.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e)
            {
                cp.removeAll();
                cp.add(p2);
                    cp.validate();
                    cp.repaint();//增加repaint()方法既可。
            }
        });

Answer 1

menuItem1.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e)
            {
                cp.removeAll();
                cp.add(p1);
                    cp.validate();
                    cp.repaint();//增加repaint()方法既可。
            }
        });
        menuItem2.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e)
            {
                cp.removeAll();
                cp.add(p2);
                    cp.validate();
                    cp.repaint();//增加repaint()方法既可。
            }
        });