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

Answer 1

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);
    }
}