Question

Im newish to java, and im writing a pong game.
Ive got the ball to move (Thats my first priority) and make it apear on screen.
But for some Reason its leaving a black line behind it, I dont know if im supposed to errase this or something, but heres the code of my 2 classes (The Ball Class is just a class to save the info of the ball)

import java.awt.Canvas;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferStrategy;
import javax.swing.JFrame;

public class Main extends Canvas implements Runnable{
    private static final long serialVersionUID = 1L;

    public static int Width=650;
    public static int Height=600;
    public boolean Running=false;
    public Thread thread;
    public Ball ball = new Ball();

    public static void main(String[] args){
      Main game = new Main();
      JFrame frame = new JFrame();
      frame.setSize(Width+25, Height+49);
      frame.setTitle("Pong By Poo");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setResizable(false);
      frame.setVisible(true);
      frame.setLocationRelativeTo(null);
      frame.add(game);
      game.start();
    }


    public void start(){
      if(Running==true){
        return;
      } else {
        Running=true;
        thread = new Thread(this);
        thread.start();
      }
    }

    public void run(){
      while(Running==true){
        try{
          Thread.sleep(5);
          Draw();
          Update();
          ball.YVelocity();
          ball.XVelocity();
        } catch(Exception e){}
      }
    }

    public void Draw(){
      BufferStrategy bs = this.getBufferStrategy();
      if(bs==null){
        createBufferStrategy(2);
      } else {
        Graphics g = bs.getDrawGraphics();
        g.setColor(Color.BLACK);
        g.fillOval(ball.BallLocationX, ball.BallLocationY, 20, 20);
        g.dispose();
        bs.show();
      }
    }

    public void Update(){
      if(ball.BallLocationX==0) {
        ball.BallMovementX=true;
        System.out.println("Ball has hit the Left");
      }
      if(ball.BallLocationX==Width) {
        ball.BallMovementX=false;
        System.out.println("Ball has hit the Right");
      }
      if(ball.BallLocationY==0){
        ball.BallMovementY=true;
        System.out.println("Ball has hit the Top");
      }
      if(ball.BallLocationY==Height){
        ball.BallMovementY=false;
        System.out.println("Ball has hit the bottom");
      }
    }
  }



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

public class Ball extends JPanel{
  public int BallLocationX;
  public int BallLocationY;
  public boolean BallMovementY; //true makes the ball go up, false makes it go down
  public boolean BallMovementX; //True makes the ball go right, false makes it go left.

  public void paintComponent(Graphics g){
    super.paintComponent(g);
    g.setColor(Color.BLACK);
    g.fillOval(BallLocationX, BallLocationY, 10, 10);
  }

  //moves the ball left to right
  public int YVelocity(){
    if(BallMovementY==true){
      BallLocationY++;
    } else {
      BallLocationY--;
    }

    return BallLocationY;
  }

  //Moves the ball up and down
  public int XVelocity(){
    if(BallMovementX==true){
      BallLocationX+=2;
    } else {
      BallLocationX-=2;
    }

    return BallLocationX;
    }
}

Please help!

Answer 1

Essentially, everywhere your ball moves stays permanently Color.BLACK on your Canvas. If you want to get rid of that, you need to refresh every time the ball moves and repaint the Canvas to Color.WHITE (or whatever) before painting the ball’s position again.

Specifically, look at your code here:

public void Draw(){
  BufferStrategy bs = this.getBufferStrategy();
  if(bs==null){
    createBufferStrategy(2);
  } else {
    Graphics g = bs.getDrawGraphics();
    g.setColor(Color.BLACK);
    g.fillOval(ball.BallLocationX, ball.BallLocationY, 20, 20);
    g.dispose();
    bs.show();
  }
}

There is no logic here that overwrites your previous changes to the Canvas indicating the ball’s location.

Also, as a side nitpick, Java standards are to have method names in camelCase.

And to answer the question in the comments: there is nothing in the Canvas API which automatically understands what you want the default background of the Canvas to be and can reset all other graphics attributes to that. To get this functionality, however, all you’ll need to do is repaint your default layout (whether it’s all one color, or a basic background image, or anything else) before painting the ball’s position on top of it.