I'm having some problems using threads in java

Question

I'm new to using threads in general can someone explain why I sometimes get this exception:

Exception in thread "Thread-3" java.lang.NullPointerException
    at Game.tick(Game.java:96)
    at Game.run(Game.java:73)
    at java.lang.Thread.run(Unknown Source)

It happens when the game starts or when I'm in the middle of playing. This error is so inconsistent as to when it decides to pop up, I have no idea what's causing it.

Here is the run and tick method:

public void run(){
    this.requestFocus();
    long lastTime = System.nanoTime();
    double amountOfTicks = 60.0;
    double ns = 100000000 / amountOfTicks;
    double delta = 0;
    long timer = System.currentTimeMillis();
    int frames = 0;
    while(isRunning) {
        long now = System.nanoTime();
        delta += (now - lastTime) / ns;
        lastTime = now;
            while(delta >= 1) {
                tick();
                //updates++;
                delta--;
            }
       render();
       frames++;

       if(System.currentTimeMillis() - timer > 1000) {
        timer += 1000;
        frames = 0;
        //updates = 0;
       }
    }
    stop();
}
public void tick(){

    for(int i = 0; i < handler.object.size(); i++){
        if(handler.object.get(i).getId() ==ID.Player){
            camera.tick(handler.object.get(0));
        }
    }
    handler.tick();
}

My apologies if this post is very similar to other posts on threads I just need somewhat of a quick fix (if there is one). I'd forgot to mention that the error is different every time the game starts up for example:

Exception in thread "Thread-3" java.lang.NullPointerException
at Enemy.tick(Enemy.java:59)
at Handler.tick(Handler.java:13)
at Game.tick(Game.java:101)
at Game.run(Game.java:73)
at java.lang.Thread.run(Unknown Source)

Exception in thread "Thread-3" java.lang.NullPointerException
at Enemy.collision(Enemy.java:105)
at Enemy.tick(Enemy.java:31)
at Handler.tick(Handler.java:13)
at Game.tick(Game.java:101)
at Game.run(Game.java:73)
at java.lang.Thread.run(Unknown Source)

This is also this is the handler class if it helps:

    import java.awt.Graphics;
import java.util.ArrayList;

public class Handler {

    ArrayList<GameObject> object = new ArrayList<GameObject>();


    public void tick(){
        for(int i = 0; i < object.size(); i++){
            GameObject tempObject = object.get(i);

            tempObject.tick();
        }
    }

    public void render(Graphics g){
        for(int i = 0; i < object.size(); i++){
            GameObject tempObject = object.get(i);

            tempObject.render(g);
        }
    }


    public void addObject(GameObject tempObject){
        object.add(tempObject);
    }

    public void removeObject(GameObject tempObject){
        object.remove(tempObject);
    }

    public void addObjectSpec(int i, GameObject tempObject){
        object.add(i, tempObject);
    }

}

What exactly is your handler object? Can you give us some more code? I am pretty sure that the `handler.object#get` method is throwing the exception. And can you also specify what code is on line **96**? — Remco Buddelmeijer, Jun 06 '18 at 23:55
The handler class stores classes with tick methods in an arraylist. When handler.tick() is executed it goes through the arraylist and executes all those other tick methods. This line 96 for(int i = 0; i < handler.object.size(); i++){. — Mike TT, Jun 07 '18 at 00:41

score -1 · Answer 1 · answered Jun 07 '18 at 00:09

Though there is not enough information to write a conclusive answer, here are some things you should check:

Make sure the ID class has a non-null field called Player
Make sure the camera is not null
Make sure the handler is not null
Make sure the handler.object is not null
Check all thread-safe objects
Make sure the handler.objects is not being manipulated while going through the loop

I'm having some problems using threads in java

1 Answers1