无法在Java中将颜色设置为JPanel

Question

I have just started with Graphics in java, and I'm already stuck. I have tried to set the color of the JPanel to red but nothing seems to work! Any help is highly appreciated.

JFrame class:

import javax.swing.JPanel;
import javax.swing.JFrame;
import java.awt.Color;


public class redBoxFrame{

    public static void main(String[]args){
        JFrame f = new JFrame();
        f.setSize(400, 200);
        f.setTitle("A red box");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel p = new redBoxPanel();
        p.setBackground(Color.RED);
        f.add(p);
        f.setVisible(true);
  }

} 

JPanel class:

   import java.awt.Graphics;
   import javax.swing.JPanel;
   import java.awt.Color;

  public class redBoxPanel extends JPanel {

     public void paintComponent(Graphics g){
     g.fillRect(0, 0, 100, 100);
     g.setColor(Color.RED);

     }
  }

As you can see I have both tried to declare the color in the JFrame class and in the JPanel class but none of them seem to work. Thank you!

Answer 1

I think you you are missing super.paintComponent(g); in your painComponent method.

Answer 2

I Believe that the solution is working however you are, like you said in your question, setting the background in the JFrame class and the JPanel class. if you remove the setBackground from the JFrame class you should see only the rectangle that you are drawing. Please try the below solution and let us know if it works.

JFrame class:

import javax.swing.JPanel;
import javax.swing.JFrame;
import java.awt.Color;


public class redBoxFrame{

    public static void main(String[]args){
        JFrame f = new JFrame();
        f.setSize(400, 200);
        f.setTitle("A red box");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel p = new redBoxPanel();
        f.add(p);
        f.setVisible(true);
  }

} 

JPanel class:

  import java.awt.Graphics;
  import javax.swing.JPanel;
  import java.awt.Color;

  public class redBoxPanel extends JPanel {

     public void paintComponent(Graphics g){
     super.paintComponent(g);
     g.fillRect(0, 0, 100, 100);
     g.setColor(Color.RED);

     }
  }

Answer 3

Everyone here seems to miss the fact that the colour should be set before drawing.

I'll set the main background to BLUE for demo purposes.

public static void main(String[] args) {
    //...
    JPanel p = new redBoxPanel();
    // BLUE bg. This covers the whole panel.
    p.setBackground(Color.BLUE);
    //...
}

And now for the red box!

public static class redBoxPanel extends JPanel {
    @Override public void paintComponent(Graphics g) {
        // You need to call the superclass' paintComponent() so that the 
        // setBackground() you called in main() is painted.
        super.paintComponent(g);

        // You need to set the colour BEFORE drawing your rect
        g.setColor(Color.RED);

        // Now that we have a colour, perform the drawing
        g.fillRect(0, 0, 100, 100);

        // More, for fun
        g.setColor(Color.GREEN);
        g.drawLine(0, 0, 100, 100);
    }
}