为什么在使用 setLayout() 时无法更改 JFRame 背景的颜色？

Question

I try to set background color to red for JFrame while keeping white color to JPanel. But it doesnt work with setLayout somehow

    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

You already have a frame (your class is extending JFrame , so it IS a JFrame ) and buildGraphics() has to build the existing frame you don't have to create a new one:

private void buildGraphics() {

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

    this.getContentPane().add(panel);

}

Answer 2

Ok, I now created another JPanel and put my first one inside to control background color

    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);