getContentPane（）。setBackground（）似乎不起作用

Question

I am trying to make a simple window appear with a red background, but it doesn't work. For some reason I keep having a grey background, while my code is this:

import java.util.*;
import java.awt.*;
import javax.swing.*;

class Courses extends JFrame {

    private Image dbImage;
    private Graphics dbg;

    Courses(){
        setTitle("Course Schedule");
        setLayout(new FlowLayout());
        getContentPane().setBackground(Color.red);
        setSize(600,600);
        setLocation(300,10);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
    }

    public void paint(Graphics g){
        dbImage = createImage(getWidth(), getHeight());
        dbg = dbImage.getGraphics();
        paintComponent(dbg);
        g.drawImage(dbImage, 0, 0, this);
    }

    public void paintComponent(Graphics g){
        g.setColor(Color.white);
        g.fillOval(100,100,100,100);
        repaint();      
    }
}

public class canvas
{
    public static void main(String[] args){
        Courses C = new Courses();
    }
}

As you can see I extend the class with Jframe and use the method getContentPane().setBackground(), so this should draw a window with a red background (and a white oval), but instead I get a grey background (with a white oval). Can someone see what goes wrong?

Also I try to use buffer painting for the first time and I am not sure if that goes well right now, but that's not important.

Answer 1

In Java, calling repaint erases all drawn elements on screen (other than Swing Components). This means that calling getContentPane().setBackground(Color.RED) is only visible until the frame is repainted (which you do right away). To prevent this, it is good practice to call super.paint(g) at the start of the paint method, as this allows the JFrame to render visuals like background color before drawing.

You could also consider using setColor(Color.RED) followed by clearRect(x, y, width, height) to change the background color. However, this is probably less efficient.