Question

How do i access some variables from another class? For example, i have 2 files:

Main.java

package keyboardgame;

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class Main extends Applet implements KeyListener
{

    private static final long serialVersionUID = 1L;
    private static boolean keyboardRightPressed = false;
    private static boolean keyboardLeftPressed = false;
    private static boolean keyboardUpPressed = false;
    private static boolean keyboardDownPressed = false;

    int ballX = 20; 
    int ballY = 20;
    int ballSpeed = 10;

    public void init()
    {
        addKeyListener(this);
    }

    public void keyPressed(KeyEvent e) 
    {
        int keyCode = e.getKeyCode();
        if(keyCode == KeyEvent.VK_RIGHT)
        {
            keyboardRightPressed = true;
            ballX = ballX + ballSpeed;
            repaint();
        }

        if(keyCode == KeyEvent.VK_LEFT)
        {
            keyboardLeftPressed = true;
            ballX = ballX - ballSpeed;
            repaint();
        }

        if(keyCode == KeyEvent.VK_UP)
        {
            keyboardUpPressed = true;
            ballY = ballY - ballSpeed;
            repaint();
        }

        if(keyCode == KeyEvent.VK_DOWN)
        {
            keyboardDownPressed = true;
            ballY = ballY + ballSpeed;
            repaint();
        }
    }

    public void keyReleased(KeyEvent e) 
    {
        int keyCode = e.getKeyCode();
        if(keyCode == KeyEvent.VK_RIGHT)
        {
            keyboardRightPressed = false;
            repaint();
        }

        if(keyCode == KeyEvent.VK_LEFT)
        {
            keyboardLeftPressed = false;
            repaint();
        }

        if(keyCode == KeyEvent.VK_UP)
        {
            keyboardUpPressed = false;
            repaint();
        }

        if(keyCode == KeyEvent.VK_DOWN)
        {
            keyboardDownPressed = false;
            repaint();
        }
    }

    public void keyTyped(KeyEvent e) {
    }


    public void paint(Graphics g)
    {
        g.fillOval(ballX,ballY,20,20);
        g.drawString("Right :"+keyboardRightPressed,0,10);
        g.drawString("Left :"+keyboardLeftPressed,0,20);
        g.drawString("Up :"+keyboardUpPressed,0,30);
        g.drawString("Down :"+keyboardDownPressed,0,40);        
    }

}

KeyThread.java

package keyboardgame;

public class KeyThread implements Runnable
{

    public KeyThread(String s,int speed)
    {
        name = s;
        time = speed;
    }

    public void run() 
    {
        try
        {
            if(keyboardRightPressed == true)
            {
                ballX = ballX + 10; 
                ballY = ballY + 10;
            }
        }
        catch(Exception e)
        {

        }
    }
}

What do i want is that from KeyThread.java to be able to modify/increment/access values from Main.java especially

int ballX = 20; 
int ballY = 20;

so i can call my thread from Main.java like

Thread t5 = new Thread(new KeyThread("Moving Right", ballX = ballX + 1, 20));
t5.start();

How can i do that?
And why is it so hard to do it?

Answer 1

If you declare your variables public instead of private, you can refer to them as (for example) Main.ballX, but this is generally a bad practice. We organize things into classes to keep separate things separate; if we start using the members of class A within class B, then class B has a dependency on A that can make it hard to debug and maintain later on. Better would be to provide a “getter” or “accessor” method inside Main (eg getBallX() and setBallX(int x)) which give the owning class (in this case, Main) better control over its private members.