Set background color in Java on a JPanel doesn't work

Question

I am working on a "paint-like" application (a little drawing software) to familiarize with Java 2D components. Here is my problem: I have a JFrame whose ContentPane is an instance of a class inheriting from JPanel. I want to set the background color to white but it remains on its default color... The class name corresponding to ContentPane is Container. Here is a simplified code:

public class Container extends JPanel {

    public Container() {
        super();
        this.setBackground(Color.WHITE);
    }
}

The JFrame constructor contains the line:

this.setContentPane(mainContainer);

Have I missed something?

Thx.

Answer 1

This could fix it...

public class Container extends JPanel
{
    public Container() 
    {
        super();
        this.setOpaque(true);
        this.setBackground(Color.WHITE);
    }
}

Answer 2

For some components, the background is switched off by default. The background color is only applied to opaque widgets. Call the following method for all components in your widget hierarchy that should paint their background:

component.setOpaque(true);

Answer 3

I also had that problem, and only that worked out is exactly what OP suggested.

// Only this works for me
this.setBackground(Color.blue);

Whole code of example class is here (just for purpose to show where I tried to put/set setBackground();

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.geom.Ellipse2D;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class CircleDraw extends JFrame {
    Float diameter = 150f;

    public CircleDraw() {
        super("Circle Draw");
        this.setSize(300, 300);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.add(new CirclePanel(diameter));
        this.setVisible(true);

        // Only this works for me
        this.setBackground(Color.blue);
    }

    public static void main(String[] args) {
        new CircleDraw();
    }
}

class CirclePanel extends JPanel {

    Float diameter;

    public CirclePanel(Float diameter) {
        super();
        // this.setOpaque(true);
        // this.setBackground(Color.WHITE);
        this.diameter = diameter;
    }

    @Override
    public void paintComponent(Graphics g) {

        int panelWidth = this.getSize().width;
        int panelHeight = this.getSize().height;

        setPreferredSize(new Dimension(300, 300));
        Graphics2D comp2D = (Graphics2D) g;
        comp2D.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        comp2D.setStroke(new BasicStroke(1f));
        // comp2D.setBackground(Color.white);
        comp2D.setPaint(Color.white);

        Ellipse2D.Float e1 = new Ellipse2D.Float((panelWidth / 2) - (diameter / 2), (panelHeight / 2) - (diameter / 2), diameter, diameter);
        comp2D.draw(e1);
    }
}