I have created a thread Variable in java and I got this error: gameThread cannot be resolved to a variable [duplicate]

Question

Hi I got an error that is about the gameThread variable in my java code is says: "Exception in thread "Thread-0" java.lang.Error: Unresolved compilation problem: gameThread cannot be resolved to a variable"

any ideas thanks for the help

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


public class GamePanel extends JPanel implements Runnable{

    public void startGameThread(){
        Thread gameThread = new Thread(this);
        gameThread.start();
    }
    @Override
    public void run(){
        while(gameThread != null){ 
            
            
        }
    }
    
}

Mateusz Grzejek · Accepted Answer · 2025-04-13 12:48:08Z

0

The problem is that gameThread is defined in function scope - that implies two things:

It is not visible outisde of the scope of this function - rest of the GamePanel class does not know anything about its existence.
It gets destroyed as soon as your function startGameThread() returns:

public void startGameThread() {
        Thread gameThread = new Thread(this);
        gameThread.start();
    } // end of function, gameThread is no longer alive

What you want is to make the thread variable a non-static class member:

public class GamePanel extends JPanel implements Runnable{

    public Thread gameThread;

    public void startGameThread() {
        gameThread = new Thread(this);
        gameThread.start();
    }
    @Override
    public void run(){
        while(gameThread != null) { 
            
            
        }
    }
}

Since startGameThread() is marked public, it is probably wise to check for scenarios where it gets called multiple times:

public void startGameThread() {
        if(gameThread == null) {
            gameThread = new Thread(this);
            gameThread.start();
        }
    }

Unless you already have control over this - then it is fine.

edited Apr 13 at 12:48

answered Apr 12 at 13:01

Mateusz Grzejek

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1 Comment

aled Apr 12 at 15:03

Using a prefix to identify members is not an accepted practice in Java coding standards. Using those may be confusing to beginners.

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