Java：更改jPanel的背景颜色

Question

I have panel1 with borderlayout . There is ball object in it, and I want to change background color with a button, but ball object doesn't allow me to change background. Why? It doesn't change, neither ball background nor panel background... How do I follow a way? Here is my code:

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;

public class project1 extends JFrame implements ActionListener{


   Color color;
  final JRadioButton sc;
  final JRadioButton bc;
  static JPanel panel1;
  JFrame frame = new JFrame("frame");
  private JPanel panel2;
  private ball b;




  public project1() {



   panel1 = new JPanel();   //panel for ball component

   panel1.setBackground(color);
   panel1.setLayout(new BorderLayout());

  //problem starts here

   panel1.add(b,BorderLayout.CENTER); 

   b.setOpaque(false); 


    panel2 = new JPanel();   //panel for button group
    panel2.setLayout(new FlowLayout());

    panel2.setBorder(BorderFactory.createLineBorder(Color.black));  //draw border for panel1 and panel 2
    panel1.setBorder(BorderFactory.createLineBorder(Color.blue));

    JButton st_btn = new JButton("start");
    JButton sp_btn = new JButton("stop");
    JButton color_btn = new JButton("color");

    sc = new JRadioButton("SC");
    bc = new JRadioButton("BC");

    final ButtonGroup bgroup = new ButtonGroup();
    bgroup.add(sc);
    bgroup.add(bc);



    sc.addActionListener(this);
    bc.addActionListener(this);
    color_btn.addActionListener(this);

    //panel2.add(Button);
    panel2.add(st_btn);
    panel2.add(sp_btn);
    panel2.add(sp_btn);
    panel2.add(color_btn);
    panel2.add(sc);
    panel2.add(bc);


    setLayout(new BorderLayout()); // frame layout

    add(panel2, BorderLayout.SOUTH);
    add(panel1, BorderLayout.CENTER);  // ball appears here


    pack();
    setResizable(false);
    setSize(500, 500);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setLocationRelativeTo(null);
    setVisible(true);

  }


  @Override
   public void actionPerformed(ActionEvent e) {

      color = JColorChooser.showDialog(this, "Select a Background Color", Color.pink);
        Object o = e.getSource();

      if (o.equals(sc)) {
          panel1.setBackground(color);

            repaint();  }

      else if (o.equals(bc)) {
           b.setColor(color);
               repaint();     }
  }



  public static void main(String[] args) {




    javax.swing.SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        project1 ex = new project1();
        ex.setVisible(true);    
      }

    });
  }


}

and this my ball class.ı allocate ball to frame within project1 class.above.

import java.awt.*;

import javax.swing.JPanel;


public class ball extends JPanel {





int x_Pos = 180;
  int y_Pos = 100;
  static Color color;  //ball color



    void up() {
    y_Pos -= 5;
    //repaint();
  }

  void down() {
    y_Pos += 5;
    //repaint();
  }

  void left() {
    x_Pos -= 5;
    //repaint();
  }

  void right() {
    x_Pos += 5;
    //repaint();
  }

  public void paintComponent(Graphics aBall) {

    super.paintComponent(aBall);
    aBall.drawOval(x_Pos, y_Pos, 190, 190);  // take fixed radius
    aBall.setColor(color);
    aBall.fillOval(x_Pos, y_Pos, 190, 190);

  }

  void setColor(Color color){
    this.color = color;


}
}

Answer 1

What's probably happening is that since you set the layout of panel to BorderLayout , any preferred size you set for ball doesn't matter, as BorderLayout doesn't respect preferred sizes. That being said, when you add ball , it extends to the size of the panel. Since JPanel is opaque by default, you can set the background to the panel , but you won't see it, because it's being covered by the ball panel. So a solution would just be to setOpaque(false) to ball .

Also, unless you don't plan on doing anything to the new ball you're adding to the panel . You may want to give it a reference.

ball someBall = new ball();
panel1.add(someBall ,BorderLayout.CENTER);

It looks like you're trying to change the background of b , which is supposedly the ball in the panel, but you add a new ball() to the panel with no reference. So if you expect whatever b is to change the color of the new ball() you added, think again. You can, with the reference above, do someBall.setBackground()

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Also you should follow Java naming convention, class names begin with capital letters
class ball → class Ball