下面這段代碼直接在JFrame上add一個button，這樣的話button是加在JFrame的哪個層次上的？為什么這樣加的話兩個按鈕是重疊在一起的，如圖所示。[code="java"]import javax.swing.*;public class ButtonTest extends JFrame{private JButton button1 = new JButton("確定");private JButton button2 = new JButton("取消");public ButtonTest(){setSize(500,500);add(button1);add(button2);}public static void main(String[] args) {ButtonTest f = new ButtonTest();f.setVisible(true);}}[/code]而如果創(chuàng)建一個JPane對象panel1，將contentPane set為panel1,在panel1上添加就得到我想要的效果了，如圖所示。[code="java"]import javax.swing.*;public class ButtonTest extends JFrame{private JButton button1 = new JButton("確定");private JButton button2 = new JButton("取消");private JPanel panel1 = new JPanel();public ButtonTest(){setSize(500,500);setContentPane(panel1);panel1.add(button1);panel1.add(button2);}public static void main(String[] args) {ButtonTest f = new ButtonTest();f.setVisible(true);}}請問這兩種方式有什么區(qū)別？