java为什么在使用setLayout（）时不能更改JFRame背景的颜色？

Question

我尝试将JFrame的背景色设置为红色，而将JPanel的背景色设置为白色。 但不知怎的，它与setLayout不兼容

    private void buildGraphics(){
    JFrame frame = new JFrame();
    setTitle("Application");
    setBounds(100, 100, 600, 400);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    getContentPane().setLayout(null);
    frame.setBackground(Color.red);
    JPanel panel = new JPanel();
    panel.setBounds(50, 50, 500, 70);
    panel.setBackground(Color.white);
    panel.setBorder(BorderFactory.createTitledBorder("Click to choose..."));
    panel.add(button1);
    panel.add(button2);
    panel.add(button3);
    getContentPane().add(panel);


}

Answer 1

好的，我现在创建了另一个JPanel，并将我的第一个放入其中以控制背景颜色

    JFrame frame = new JFrame();
    setTitle(" Application");
    setBounds(100, 100, 600, 400);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    getContentPane().setLayout(null);

    JPanel panelMain = new JPanel();
    panelMain.setBounds(5, 5, 595, 395);
    panelMain.setBackground(Color.red);
    getContentPane().add(panelMain);


    JPanel panel = new JPanel();
    panel.setBounds(50, 50, 500, 70);
    panel.setBackground(Color.white);
    panel.setBorder(BorderFactory.createTitledBorder("What would you like to do?"));
    panel.add(button1);
    panel.add(button2);
    panel.add(button3);
    panelMain.add(panel);