将Jpanel添加滚动条，无法动态使JPanel变大
这段程序本意是想点击按钮，让panel变大，然后出现滚动条

Java code

public class MyFrame extends JFrame {

    private static final long serialVersionUID = 1L;
    
    private JPanel panel;
    private JButton button;
    
    public MyFrame()
    {
        setSize(new Dimension(400,300));
        setLayout(new BorderLayout());
        panel=new JPanel();
        JScrollPane js=new JScrollPane(panel);
        panel.setBackground(Color.RED);
        getContentPane().add(js,BorderLayout.NORTH);
        button=new JButton("点击变大");
        button.setSize(100, 50);
        add(button,BorderLayout.SOUTH);
        button.addActionListener(new ActionListener(){

            public void actionPerformed(ActionEvent arg0) {
                int x=MyFrame.this.getPanel().getWidth();
                int y=MyFrame.this.getPanel().getHeight()+20;
                MyFrame.this.getPanel().setSize(x, y);
                MyFrame.this.repaint();
            }
            
        });
        getContentPane().add(button,BorderLayout.SOUTH);
        setVisible(true);
    }
    
    public JPanel getPanel()
    {
        return this.panel;
    }
    
    public static void main(String[] args)
    {
        new MyFrame();
    }
}

------解决方案--------------------
加个JScrollPane，把JPanel放进去
------解决方案--------------------
用setPreferredSize(Dimension preferredSize)
------解决方案--------------------
试试这个，应该好了:
public class MyFrame extends JFrame {

private static final long serialVersionUID = 1L;

private JPanel panel;
private JButton button;

public MyFrame()
{
setSize(new Dimension(400,300));
setLayout(new BorderLayout());
panel=new JPanel();
JScrollPane js=new JScrollPane(panel);
panel.setBackground(Color.RED);
// js.setPreferredSize(new Dimension(100,80));
getContentPane().add(js,BorderLayout.CENTER);
button=new JButton("Zoom in");
add(button,BorderLayout.SOUTH);
button.addActionListener(new ActionListener(){

public void actionPerformed(ActionEvent arg0) {
int x=MyFrame.this.getPanel().getWidth();
int y=MyFrame.this.getPanel().getHeight()+20;
getPanel().setPreferredSize(new Dimension(x, y));
getPanel().setSize(getPanel().getPreferredSize());
//MyFrame.this.repaint();
}

});
getContentPane().add(button,BorderLayout.SOUTH);
setVisible(true);
}

public JPanel getPanel()
{
return this.panel;
}

public static void main(String[] args)
{
new MyFrame();
}
}
------解决方案--------------------
还有，记的区别setSize和setPreferredSize。
这里要使用setPreferredSize。 不多说了，请自己去看一下源代码。这些细节的区别文档给出的不多。