Question

I’m trying to create simple Pong game.
The first thing I’m working on is getting a paddle to move left or right when the user clicks the “Left” or “Right” button provided in the frame.
When the user clicks a button, an action listener in the MainPanel class changes the constant in the Paddle class called “paddlePosition” and then calls “repaint()”.
However, it’s not working, and the paddle is not moving. I know the buttons are working because it outputs “Hello World” and the value of paddlePosition but apparently something is wrong with the repaint.
I’m using Swing, JFrame, and Graphics2D to get everything working so far.

MY CLASSES:

Pong Frame (run class)

import java.awt.BorderLayout;
import javax.swing.JButton;
import javax.swing.JFrame;


public class PongFrame extends JFrame{

    private static final long serialVersionUID = 1L;

    public PongFrame(){

    ButtonPanel buttonPanel = new ButtonPanel();
    MainPanel mainPanel = new MainPanel();

    add(buttonPanel, BorderLayout.SOUTH);
    add(mainPanel);

    setSize(400, 400);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setLocationRelativeTo(null);
    setVisible(true);
}

    public static void main(String[] args) {
        new PongFrame();

    }

}

Main Panel:

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class MainPanel extends JPanel implements ActionListener{

    public Paddle paddle;

    public MainPanel(){

        setSize(300, 300);
        paddle = new Paddle();
    }

    public void paint(Graphics g) {
        super.paint(g);
        Graphics2D g2 = (Graphics2D)g;
        paddle.draw(g2);
    }

    public void actionPerformed(ActionEvent e) {
        System.out.println("Hello World");
        Paddle.movePaddleLeft();
        System.out.println(Paddle.paddlePosition);
        this.repaint();
    }
}

Button Panel:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JPanel;

public class ButtonPanel extends JPanel {


    private static final long serialVersionUID = 1L;

    public MainPanel mainPanelActionListener = new MainPanel(); 

    JButton left;
    JButton right;

    public ButtonPanel(){
        left = new JButton("Left");
        right = new JButton("Right");

        add(left);
        add(right);

        left.addActionListener(mainPanelActionListener);
        right.addActionListener(mainPanelActionListener);
    }
}

Paddle:

import java.awt.Graphics2D;

public class Paddle{

    public static int paddlePosition = 200;
    private java.awt.Rectangle paddle;
    private int centerCoordinateX;
    private int centerCoordinateY;

    public Paddle(){
        //paddle = new java.awt.Rectangle(paddlePosition, 285, 100, 10); // x, y, width, height
    }

    public void draw(Graphics2D g2){
        paddle = new java.awt.Rectangle(paddlePosition, 285, 100, 10);
        g2.draw(paddle);
    }

    public static void movePaddleLeft(){
        paddlePosition = (paddlePosition + 3);
    }
}

Does anyone see what is wrong? Why won’t the paddle move when the buttons are clicked?

Answer 1

In the ButtonPanel you’re creating a new MainPanel. This causes you to create a new invisible panel that you’re updating.

What you actually need to do, is send the MainPanel to your ButtonPanel in it’s constructor.

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Edit: (disclaimer: all code here is generated without an IDE)

In the PongFrame constructor do the following:

MainPanel mainPanel = new MainPanel();
ButtonPanel buttonPanel = new ButtonPanel(mainPanel);

This creates a ButtonPanel with a reference to the MainPanel.

Now for the ButtonPanel:

public MainPanel mainPanelReference;

JButton left;
JButton right;

public ButtonPanel(MainPanel mainPanelReference) {
    this.left = new JButton("Left");
    this.right = new JButton("Right");

    add(left);
    add(right);

    this.mainPanelReference = mainPanelReference;

    left.addActionListener(mainPanelReference);
    right.addActionListener(mainPanelReference);
}