JFrame和JPanel的不同背景颜色

Question

I want to draw a JPanel on a JFrame. Background color for JFrame is different for JPanel. So far, this my code:

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

public class DifferentColor extends JFrame{

JPanel p;

GradientColor(){
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.setPreferredSize(new Dimension(500, 500));
    this.getContentPane().setBackground(Color.yellow);        
    p = new JPanel();
    p.setPreferredSize(new Dimension(400, 400));
    p.setBackground(Color.red);
    this.add(p);
    this.pack();
    this.setVisible(true);
  }

  public static void main(String[] args) {
    // TODO code application logic here
      new DifferentColor ();
  }
}

WhenI run the code the color is red. Is not red (JPanel) on yellow (JFrame). How to solve it ?

Answer 1

Your problem is that the JPanel has the same size then your JFrame . The reason was explained by Arvind.

Following snippet would assign the JPanel to the North region and add a thick blue border around it for demonstration.

public void showFrame() {
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.setPreferredSize(new Dimension(500, 500));
    this.getContentPane().setBackground(Color.yellow);
    JPanel p = new JPanel();
    p.setPreferredSize(new Dimension(400, 400));
    p.setBackground(Color.red);
    Border border = BorderFactory.createLineBorder(Color.blue, 10);
    border.isBorderOpaque();
    p.setBorder(border);
    this.add(p, BorderLayout.NORTH);
    this.pack();
    this.setVisible(true);
}

public static void main(String[] args) {
    new DifferentColor().showFrame();
}

Have a look also in the Swing tutorial about use of panels .