Question

MyActivity implements a CustomListener defined in MyClass.java. I have a function defined in MyClass that should trigger the listener and do some action(finish() MyActivity) defined in the activity. Somehow, I am getting null reference to the listener. Where am I going wrong?

Please refer to the code below!

MyActivity.java

import com.project.MyClass.CustomListener;

public class MyActivity extends Activity implements CustomListener {

    Context context;
    CustomListener listener;

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        context = this;

        myClass = new MyClass(context);
        myClass.setOnCustomListener(listener);

    }

    public void doThisWhenTriggered()
    {
    ...
    }

}

MyClass.java

public class MyClass{

    private Contect ctx;
    ArrayList<CustomListener> listeners = new ArrayList<CustomListener>();

    public MyClass(Context context)
    {
        super();
        this.ctx = context
    }

public interface CustomListener
{
       public void doThisWhenTriggered();
}


public void setOnCustomListener(CustomListenerListener listener)
{
    this.listeners.add(listener);
}

    public void generateTrigger()
    {
        CustomListener listener = listeners.get(0);

        if(listener != null)
            listener.doThisWhenTriggered();
        else
            Log.d("MyAPP", "Listener is NULL");
    }

}

MyBroadcastReceiver.java

public void onReceive(Context context, Intent intent) {
    ...
    callMyClass(Context context)
}

public void callMyClass(Context context)
{
    MyClass myClass= new myClass(context);
    myClass.generateTrigger(context);
}

EDIT

The problem was BECAUSE of accessing different instances of MyClass in BroadcastReceiver and Listeners list as correctly pointed out by @Binyamin Sharet

The solution is to declare the listener list as static and also public void generateTrigger() as static so that the receiver’s callMyClass looks like :

public void callMyClass(Context context)
{
    MyClass.generateTrigger(context);
}

Answer 1

You never instantiate listener (declared as CustomListener listener;) and therefore its always null, you just need to set the activity as the listener, as it implements the interface:

myClass.setOnCustomListener(this);

As seen in your code, you create a new instance of the class in the receiver, so the listener you set does not exist in the listeners list of new instance, since the list is not static.