为什么JFrame只能放入两个JPanel
代码如下，不知道为什么，只能显示第一个jpanel和最后一个jpanel，这是为什么，高手帮帮忙

package swing;

import java.awt.Color;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class test2 extends JFrame{

/**
* 
*/
private static final long serialVersionUID = 1L;
private JFrame frame;

public test2() {
// TODO Auto-generated constructor stub
frame = new JFrame();
//panel=(JPanel) frame.getContentPane();
//JPanel panel=new JPanel();
//panel.setLayout(new FlowLayout());
//panel.setOpaque(false);

JPanel jPanel=new JPanel();
JPanel jPanel2=new JPanel();
JPanel jPanel3=new JPanel();
JPanel jPanel4=new JPanel();

jPanel.setLayout(null);
jPanel2.setLayout(null);
jPanel3.setLayout(null);
jPanel4.setLayout(null);

jPanel.setSize(500, 200);
jPanel2.setSize(500, 200);
jPanel3.setSize(500, 200);
jPanel4.setSize(500, 100);

//jPanel.setBounds(100, 0, 100, 20);
JButton jButton=new JButton("第一个jpanel");
JButton jButton2=new JButton("第二个jpanel");
JButton jButton3=new JButton("第三个jpanel");
JButton jButton4=new JButton("第四个jpanel");

jButton.setBounds(0, 0, 200, 20);
jButton2.setBounds(0, 200, 200, 20);
jButton3.setBounds(0, 400, 200, 20);
jButton4.setBounds(0, 600, 200, 20);

jPanel.add(jButton);
jPanel2.add(jButton2);
jPanel3.add(jButton3);
jPanel4.add(jButton4);

jPanel.setBackground(Color.blue);
jPanel2.setBackground(Color.white);
jPanel3.setBackground(Color.yellow);
jPanel4.setBackground(Color.gray);

frame.add(jPanel);
frame.add(jPanel2);
frame.add(jPanel3);
frame.add(jPanel4);

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(500, 700);
frame.setVisible(true);


}

public static void main(String[] args) {
new test2();
}

}


------解决方案--------------------

Java code

package novemberTestDay;

import java.awt.BorderLayout;
import java.awt.EventQueue;
import javax.swing.JButton;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Test extends JFrame {

    /**
     * Launch the application
     * @param args
     */
    public static void main(String args[]) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Test frame = new Test();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the frame
     */
    public Test() {
        super();
        setBounds(100, 100, 500, 375);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        final JPanel panel = new JPanel();
        getContentPane().add(panel, BorderLayout.NORTH);

        final JPanel panel_1 = new JPanel();
        panel.add(panel_1);

        final JButton button = new JButton();
        button.setText("New JButton");
        panel_1.add(button);

        final JPanel panel_2 = new JPanel();
        panel.add(panel_2);

        final JButton button_1 = new JButton();
        button_1.setText("New JButton");
        panel_2.add(button_1);

        final JPanel panel_3 = new JPanel();
        panel.add(panel_3);

        final JButton button_2 = new JButton();
        button_2.setText("New JButton");
        panel_3.add(button_2);

        final JPanel panel_4 = new JPanel();
        panel.add(panel_4);

        final JButton button_3 = new JButton();
        button_3.setText("New JButton");
        panel_4.add(button_3);
        //
    }

}