Java non-static variable

Question

I am trying to create ball collisions using two ball sprites. When one ball collides its speed should depend on the other balls initial speed, so in the sprites I need to call in the speeds of the other ball, but I can't figure out how to get past the non-static variable message. This is the line with the error

xa = BIGBALLspeedx()*(BIGBALLmass()-1)+(BallTest.ball.BALLspeedx()*2*1);

Here is my code for one of the sprites and the main package 'BallTest':

package balltest;

import java.awt.Graphics2D;
import java.awt.Rectangle;
import static java.lang.Math.sqrt;

public class BigBall {

    private final int DIAMETER = 80;
    int x = (int )(Math.random() * 200 + 15 );
    int y = (int )(Math.random() * 200 + 15 );
    int xa = (int )(Math.random() * 4  );
    int ya = (int )(1 );

    private BallTest game;

    public BigBall(BallTest game) {
        this.game= game;
    }

    public void  move() {
        if (x + xa < 0)
            xa = 1;
        if (x + xa > game.getWidth() - DIAMETER)
            xa = -1;
        if (y + ya < 0)
            ya = 1;
        if (y + ya > game.getHeight() - DIAMETER)
            ya=-1;
        if (y + ya < 1)
            ya=1;
        if (50>sqrt(xDIFF()*xDIFF()+yDIFF()*yDIFF()) && -50<sqrt(xDIFF()*xDIFF()+yDIFF()*yDIFF())) {
            xa = BIGBALLspeedx()*(BIGBALLmass()-1)+(BallTest.ball.BALLspeedx()*2*1);
        }
        x = x + xa;
        y = y + ya;
    }

    public int BIGBALLx() {
        return x+40;
    }

    public int BIGBALLy(){
        return y+40;
    }

    public int BIGBALLspeedx(){
        return xa;
    }

    public int BIGBALLspeedy(){
        return ya;
    }

    public int BIGBALLmass(){
        return 5;
    }

    public int xDIFF(){
        return game.ball.BALLx()-game.BigBall.BIGBALLx();
    }

    public int yDIFF(){
        return game.ball.BALLy()-game.BigBall.BIGBALLy();
    }

    public void paint2(Graphics2D g) {
        g.fillOval(x, y, DIAMETER, DIAMETER);
    }

    public Rectangle getBounds() {
        return new Rectangle(x, y, DIAMETER, DIAMETER);
    }
}

Here's BallTest code:

package balltest;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import static java.awt.image.ImageObserver.ABORT;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;

/**
 *
         * @author F27834
 */
public class BallTest extends JPanel {

    Ball ball = new Ball(this);
    BigBall BigBall = new BigBall(this);

    private void move() {
        ball.move();
        BigBall.move();
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);
        g.setColor(Color.RED);
        Graphics2D g2d = (Graphics2D) g;
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);

        ball.paint(g2d);

        g.setColor(Color.BLUE);
        Graphics2D g3d = (Graphics2D) g;
        BigBall.paint2(g3d);
    }

    public void gameOver() {
        JOptionPane.showMessageDialog(this, "Game Over", "Game Over", JOptionPane.YES_NO_OPTION);
        System.exit(ABORT);
    }

    public static void main(String[] args) throws InterruptedException {
        JFrame frame = new JFrame("Mini Tennis");
        BallTest game = new BallTest();
        frame.add(game);
        frame.setSize(300, 400);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        while (true) {
            game.move();
            game.repaint();
            Thread.sleep(10);
        }
    }
}

Answer 1

You are trying to reference BallTest.ball - BallTest is presumably another class, while ball is an instance variable defined in that class. In Java terms, BallTest is a static reference (it refers to a class, not an object), while ball is only defined of an instance of BallTest.

In other parts of your code, you refer to game.ball. It's impossible to tell for sure without seeing the rest of the code, but should you be using game.ball instead of BallTest.ball?

That is, should it be like this?

xa = BIGBALLspeedx()*(BIGBALLmass()-1)+(game.ball.BALLspeedx()*2*1);