为什么JFrame中只能显示一个控件？

Question

因为你使用的是BorderLayout，在BorderLayout被使用的情况下，调用rootFrame.getContentPane().add时，如果没有指定安放的位置，则会默认把空间放在中央并伸展至四周，所以你可以指定排放的位置，如NORTH, SOUTH, CENTER, EAST, WEST等，也可以改用别的Layout，如FlowLayout。

Answer 1

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
public class SwingSample implements ActionListener
{
        private JFrame rootFrame=null;
        private JButton exitButton=null;
        private JButton clickMeButton=null;
        private JLabel rootLabel=null;
        private JLabel  clickNumsLabel=null;
        private int m_iClickNum=0;
        private String m_strLabelPrefix="一共点击了：";
        private String m_strLabelPostfix="次";
        private Hashtable     componentTable;
        public SwingSample()
        {
                componentTable=new Hashtable(10);
                rootFrame=new JFrame();
                rootLabel=new JLabel();
                rootFrame.getContentPane().setLayout(new FlowLayout());///////////////在这里
                rootFrame.setDefaultLookAndFeelDecorated(true);
                //添加“退出”按纽
                exitButton=new JButton("退出");
                exitButton.setMnemonic(KeyEvent.VK_Q);
                exitButton.addActionListener(this);
                //
                clickMeButton =new JButton("点击");
                clickMeButton.setMnemonic(KeyEvent.VK_C);
                clickMeButton.addActionListener(this);
                //
                clickNumsLabel=new JLabel(m_strLabelPrefix +m_iClickNum+m_strLabelPostfix);                //                rootFrame.getContentPane().add(exitButton);
                rootFrame.getContentPane().add(clickMeButton);
                rootFrame.getContentPane().add(clickNumsLabel);
                rootFrame.pack();
                rootFrame.setVisible(true);                componentTable.put("RootFrame",rootFrame);
                componentTable.put("ExitButton",exitButton);
        }
        //
        //
        //
        private JFrame getRootFrame()
        {
                return rootFrame;
        }        //
        public static void main(String[] args)
        {
                SwingSample theApp=new SwingSample();
        }
        //
        public void actionPerformed(ActionEvent e)
        {
                if(e.getSource()==exitButton)
                {
                        System.exit(0);
                }
                else
                if(e.getSource()==clickMeButton)
                {
                        m_iClickNum+=1;
                        clickNumsLabel.setText(m_strLabelPrefix+m_iClickNum+m_strLabelPostfix);
                }
        }
}