Question

I’m writing a little Android game. It should display a random sequence of 4 animals which i implemented as ImageButtons. The user has to remember this sequence and repeat it afterwards.

My problem now is the right timing how the Imagebuttons get visible.

I got the following NullPointerException and couldn’t figure out why. Maybe anyone can help!?

Heres my Main Activity:

package lichtenberger.paul;

import java.util.Random;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.ImageButton;
import android.widget.TextView;

public class Game extends Activity {

    int Reihenfolge[] = new int[40];

    Random generator = new Random();


    public final int CAT = 0;
    public final int MAN = 1;
    public final int BIRD = 2;
    public final int SHEEP = 3;
    public Handler handler;
    public Thread AnimalThread;
    {for(int i = 0; i<40; i++)Reihenfolge[i]=generator.nextInt(4);}


    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_game);

        final ImageButton cat = (ImageButton)findViewById(R.id.catButton);
        final ImageButton sheep = (ImageButton)findViewById(R.id.sheepButton);
        final ImageButton man = (ImageButton)findViewById(R.id.manButton);
        final ImageButton bird = (ImageButton)findViewById(R.id.birdButton);
        final TextView score = (TextView)findViewById(R.id.scoreNTV);

        handler = new Handler(){

            @Override

            public void handleMessage(Message msg){

                switch (msg.what) {
                case 0:
                    cat.setVisibility(1);
                    man.setVisibility(0);
                    bird.setVisibility(0);
                    sheep.setVisibility(0);
                    break;
                case 1:
                    man.setVisibility(1);
                    bird.setVisibility(0);
                    sheep.setVisibility(0);
                    cat.setVisibility(0);
                    break;
                case 2:
                    bird.setVisibility(1);
                    sheep.setVisibility(0);
                    cat.setVisibility(0);
                    man.setVisibility(0);
                    break;
                case 3:
                    sheep.setVisibility(1);
                    cat.setVisibility(0);
                    man.setVisibility(0);
                    bird.setVisibility(0);
                    break;
                }

            }
        };          
            ShowSequence show = new ShowSequence();
            Thread showSeq = new Thread(show);
            showSeq.start();

        };

}

My Thread Class:

package lichtenberger.paul;   

public class ShowSequence extends Game implements Runnable{    
    @Override
    public void run() {                 
        show();
    }

    private void show() {

        for(int i = 0; i<40; i++){
        switch (Reihenfolge[i]) {
        case 0:
                try {
                // this pauses the Thread: Alternative to doing stuff...
                Thread.sleep(2000);

                handler.sendEmptyMessage(CAT);
                } catch (InterruptedException e) {
                e.printStackTrace();}
                break;

        case 1:
            try {
                // this pauses the Thread: Alternative to doing stuff...
                Thread.sleep(2000);

                handler.sendEmptyMessage(MAN);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            break;

        case 2:
            try {
                // this pauses the Thread: Alternative to doing stuff...
                Thread.sleep(2000);

                handler.sendEmptyMessage(BIRD);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            break;

        case 3: 
            try {
                // this pauses the Thread: Alternative to doing stuff...
                Thread.sleep(2000);

                handler.sendEmptyMessage(SHEEP);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        default:
            break;
        }
    }
}}

and my LogCat:

12-20 15:20:34.974: ERROR/AndroidRuntime(598): FATAL EXCEPTION: Thread-75
12-20 15:20:34.974: ERROR/AndroidRuntime(598): java.lang.NullPointerException
12-20 15:20:34.974: ERROR/AndroidRuntime(598):     at lichtenberger.paul.ShowSequence.show(ShowSequence.java:50)
12-20 15:20:34.974: ERROR/AndroidRuntime(598):     at lichtenberger.paul.ShowSequence.run(ShowSequence.java:10)
12-20 15:20:34.974: ERROR/AndroidRuntime(598):     at java.lang.Thread.run(Thread.java:856)

EDIT:

the whole code in one Activity which didnt work:

package lichtenberger.paul;

import java.util.Random;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.ImageButton;
import android.widget.TextView;

public class Game extends Activity {

    int Reihenfolge[] = new int[40];

    Random generator = new Random();
    public ImageButton cat;
    public ImageButton man;
    public ImageButton bird;
    public ImageButton sheep;
    public TextView score;
    private static Handler handler;
    public final int CAT = 0;
    public final int MAN = 1;
    public final int BIRD = 2;
    public final int SHEEP = 3;
    public Runnable showAnimal;
    public Thread AnimalThread;
    {for(int i = 0; i<40; i++)Reihenfolge[i]=generator.nextInt(4);}


    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_game);

        setupUI();
        showSequence();
        initHandler();
    }



    public void showSequence() {

                showAnimal = new showAnimal();
                AnimalThread = new Thread(showAnimal);
                AnimalThread.start();


    }



    public void setupUI() {

        cat = (ImageButton)findViewById(R.id.catButton);
        sheep = (ImageButton)findViewById(R.id.sheepButton);
        man = (ImageButton)findViewById(R.id.manButton);
        bird = (ImageButton)findViewById(R.id.birdButton);
        score = (TextView)findViewById(R.id.scoreNTV);

    }

    private void initHandler() {
        handler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                switch (msg.what) {
                case CAT:
                    cat.setVisibility(1);
                    man.setVisibility(0);
                    bird.setVisibility(0);
                    sheep.setVisibility(0);
                    break;
                case MAN:
                    man.setVisibility(1);
                    bird.setVisibility(0);
                    sheep.setVisibility(0);
                    cat.setVisibility(0);
                    break;
                case BIRD:
                    bird.setVisibility(1);
                    sheep.setVisibility(0);
                    cat.setVisibility(0);
                    man.setVisibility(0);
                    break;
                case SHEEP:
                    sheep.setVisibility(1);
                    cat.setVisibility(0);
                    man.setVisibility(0);
                    bird.setVisibility(0);
                    break;
                }
            }
        };

    }

    class showAnimal implements Runnable {
        public void run() {
            show();
        }

        private void show() {

            for(int i = 0;i<40;i++){

            switch (Reihenfolge[i]) {

            case 0:
                try {
                    // this pauses the Thread: Alternative to doing stuff...
                    Thread.sleep(2000);

                    handler.sendEmptyMessage(CAT);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                break;

            case 1:
                try {
                    // this pauses the Thread: Alternative to doing stuff...
                    Thread.sleep(2000);

                    handler.sendEmptyMessage(MAN);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                break;

            case 2:
                try {
                    // this pauses the Thread: Alternative to doing stuff...
                    Thread.sleep(2000);

                    handler.sendEmptyMessage(SHEEP);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                break;
            case 3:
                try {
                    // this pauses the Thread: Alternative to doing stuff...
                    Thread.sleep(2000);

                    handler.sendEmptyMessage(BIRD);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                break;
            }
        }
    }



}
}

Thanks for help 😉

Answer 1

The problem is that you have 2 separate objects – instance of Game that has non-null handler. And instance of ShowSequence with null handler. You shouldn’t inherit ShowSequecne from Game. It doesn’t get all Game‘s variables automically, you should make ShowSequence inner class of in Game as @harism suggested.
Or you have to set all variables in ShowSequence explicitly in constructor or immediately after you created it.