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

问题

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.

回答1:

This could fix it...

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

回答2:

Some components' 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 hierarchie that should paint its background:

c.setOpaque(true);

回答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);
    }
}

来源：https://stackoverflow.com/questions/17966773/set-background-color-in-java-on-a-jpanel-doesnt-work

标签

java

jpanel