Question

My situation is that I want to call a particular method to get called from ClassA from inside my ClassB which extends the BroadcastReceiver class. I understand how to do this from inside another class file etc and how to setup the BroadcastReceiver, or so I thought.

Below is the class file that extends the BroadcastReceiver.
public class WatchPhone extends BroadcastReceiver {

private ClassA classA;

@Override
public void onReceive(Context context, Intent BackToMain) {
    // TODO Auto-generated method stub
    TelephonyManager telephony = (TelephonyManager)context.getSystemService(Context.TELEPHONY_SERVICE); 
    int state = telephony.getCallState(); 
    switch(state) { 
        case TelephonyManager.CALL_STATE_RINGING:
            Log.d("TestServiceReceiver", "IDLE"); 
            classA.IwantToDoThis();
            break; 
    }
}

The method that I am trying to call does work great in the application if I manually request it.
In my AndroidManifest.xml I added this

    <uses-permission android:name="android.permission.READ_PHONE_STATE"/>

and this at the bottom just above the closing application tag.

            <receiver android:name=".WatchPhone">
                    <intent-filter> 
                    <action android:name="android.intent.action.PHONE_STATE" /> 
                    </intent-filter> 
            </receiver>

When the phone rings, the application crashes then closes. Any ideas on what I should do.

Answer 1

I can see one problem immediately: you declared an instance of ClassA but did not instantiate it. That means classA = null and you are probably getting a NullPointerException when you use it.

Try this instead:

private ClassA classA = new ClassA();

Alternately, you can instantiate it near the top of onReceive():

classA = new ClassA();