Question

I’m trying to do a Java Pong game. I have very little experience with Java and need some help. I’ve read a lot of stuff online, and below is the code I’ve come up with.
I’ve created the following classes, but all I get is a grey screen. I’m just trying to implement the simplest solution for this. Could anyone point out any issues this code has? Thank you.

    package pong_test_1;

    import javax.swing.*;
    import java.awt.*;
    import java.awt.event.KeyEvent;
    import java.awt.event.KeyListener;

    /**
     *
     * @author jtrinidad
     */

    public class main_class extends JFrame implements KeyListener{
        static final int FRAME_WIDTH = 800;
        static final int FRAME_HEIGHT = 500;
        JFrame f;

        public main_class()
        {
            super();
            addKeyListener (this);
            setFocusable (true);

            f = new JFrame("Pong");
            f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

            f.setResizable(false);

            f.setSize(FRAME_WIDTH,FRAME_HEIGHT);
            f.setVisible(true);

        }

        @Override
         public void keyTyped(KeyEvent e) {
        }
        @Override
        public void keyPressed(KeyEvent e) {
            Pong_Test_1 p = new Pong_Test_1();
            int keyCode = e.getKeyCode();
            switch( keyCode ) { 
                case KeyEvent.VK_UP:
                    Pong_Test_1.p2_pos_y--; 
                    break;
                case KeyEvent.VK_DOWN:
                    Pong_Test_1.p2_pos_y++; 
                    break;
                case KeyEvent.VK_Q:
                    Pong_Test_1.p1_pos_y++; 
                    break;
                case KeyEvent.VK_A :
                    Pong_Test_1.p1_pos_y++; 
                    break;
            }
            add(p);
            repaint();
        }

        /** Handle the key released event from the text field. */
         @Override
        public void keyReleased(KeyEvent e) {

        }

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


    }

and

    /*
     * To change this template, choose Tools | Templates
     * and open the template in the editor.
     */
    package pong_test_1;

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



    /**
     *
     * @author jtrinidad
     */
    public class Pong_Test_1 extends JPanel implements ActionListener{

        /**
         * @param args the command line arguments
         */
        //Sizes of each object
        static final int WIDTH = 10;
        static final int HEIGHT = 120;
        static final int RADIUS = 10;
        //Position of Player 1's paddle
        static int p1_pos_x = 10;
        static int p1_pos_y = main_class.FRAME_HEIGHT/2;
        //Position of Player 2's paddle
        static int p2_pos_x = main_class.FRAME_WIDTH - 20;
        static int p2_pos_y = main_class.FRAME_HEIGHT/2;;
        //Position of the ball
        static int ball_pos_x;
        static int ball_pos_y;


        Timer animator;

        public Pong_Test_1()
        {
            animator = new Timer (10, this);
            animator.start();
        }


        public void paintComponent(Graphics g){
            super.paintComponent(g);
            this.setBackground(Color.BLACK); //Sets background color

            g.setColor(Color.WHITE);
            g.fillRect(p1_pos_x, p1_pos_y, WIDTH, HEIGHT);
            g.fillRect(p2_pos_x, p2_pos_y, WIDTH, HEIGHT);
            g.fillOval(100, 100, RADIUS, RADIUS);
        }

        public void actionPerformed( ActionEvent e ) {
            repaint();
            revalidate();  // new line
        }

        private void addKeyListener(Pong_Test_1 aThis) {
            throw new UnsupportedOperationException("Not yet implemented");
        }


    }

Answer 1

See if this SSCCE works more to your expectations, the main problem was both extending and having an instance of a JFrame.

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

public class main_class implements KeyListener {

    static final int FRAME_WIDTH = 800;
    static final int FRAME_HEIGHT = 500;
    JFrame f;

    public main_class() {
        f = new JFrame("Pong");

        JPanel gui = new JPanel();
        gui.setFocusable(true);
        gui.addKeyListener(this);
        f.setContentPane(gui);

        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        f.setResizable(false);

        f.setSize(FRAME_WIDTH, FRAME_HEIGHT);
        f.setVisible(true);
    }

    @Override
    public void keyTyped(KeyEvent e) {
    }

    @Override
    public void keyPressed(KeyEvent e) {
        // this will steal the focus for KeyEvents, or rather..
        // Pong_Test_1 p = new Pong_Test_1();
        int keyCode = e.getKeyCode();
        System.out.println(e);
        switch (keyCode) {
            case KeyEvent.VK_UP:
                Pong_Test_1.p2_pos_y--;
                break;
            case KeyEvent.VK_DOWN:
                Pong_Test_1.p2_pos_y++;
                break;
            case KeyEvent.VK_Q:
                Pong_Test_1.p1_pos_y++;
                break;
            case KeyEvent.VK_A:
                Pong_Test_1.p1_pos_y++;
                break;
        }
        // ..adding it will steal the content area & focus
        //add(p);
        f.repaint();
    }

    /**
     * Handle the key released event from the text field.
     */
    @Override
    public void keyReleased(KeyEvent e) {
    }

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

As general advice:

• Don’t extend frame, simply keep a reference to one.
• For Swing, use Key Bindings rather than a KeyListener.