Question

I am developing an android application. My question is can I get the application context in an abstract class? it gives me null pointer exception for the context variable and I assume it is maybe because I am using it inside an abstract class.

NOTE: In my code I have an abstract class (the abstract class itself extends Activity) that I use for instantiating other classes – Activites. In abstract class i create an instance of an activity helper, a class that handles some common activity-related functionality in the app. The is my activity helper class:

public class ActivityHelper { 
protected Activity mActivity; 

public static ActivityHelper createInstance(Activity activity, Context context) { 
return UIUtils.isTablet(context) ? 
new ActivityHelperTablet(activity) : 
new ActivityHelper(activity); 
} 

protected ActivityHelper(Activity activity) { 
mActivity = activity; 
}  
     }

Where the function isTablet receives a context variable that I create in my abstract class

     public static boolean isTablet(Context context) {
      // Can use static final constants like HONEYCOMB, declared in later versions 
      // of the OS since they are inlined at compile time. 
     return isHoneycomb() ||    
    ((context.getResources().getConfiguration().screenLayout    &        
    Configuration.SCREENLAYOUT_SIZE_MASK) >= Configuration.SCREENLAYOUT_SIZE_LARGE);
    } 

the important part of BaseActivity:

public abstract class BaseActivity extends Activity { 

Context context = this.getApplicationContext(); 
final ActivityHelper mActivityHelper = ActivityHelper.createInstance(this, context);
    }

But it gives me a null pointer exception. So I thought that it’s maybe because of the abstract class.
And this is the log:

06-12 15:59:56.011: E/AndroidRuntime(1206): Caused by: java.lang.NullPointerException
06-12 15:59:56.011: E/AndroidRuntime(1206):     at   
android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:101)
06-12 15:59:56.011: E/AndroidRuntime(1206):     at   
com.navayo.sec.voip.activities.BaseActivity.<init>(BaseActivity.java:35)
06-12 15:59:56.011: E/AndroidRuntime(1206):     at 
com.navayo.sec.voip.activities.MainActivity.<init>(MainActivity.java:43)
06-12 15:59:56.011: E/AndroidRuntime(1206):     at 
java.lang.Class.newInstanceImpl(Native Method)
06-12 15:59:56.011: E/AndroidRuntime(1206):     at   
java.lang.Class.newInstance(Class.java:1319)
06-12 15:59:56.011: E/AndroidRuntime(1206):     at 
android.app.Instrumentation.newActivity(Instrumentation.java:1023)
06-12 15:59:56.011: E/AndroidRuntime(1206):     at   
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1870)

Answer 1

Having an abstact class doesn’t matter.

You have a null pointer because your context is null, which means somewhere in your methods there is an error, without the code to review i can’t help you more.

EDIT:

public static ActivityHelper createInstance(Activity activity) { 
return UIUtils.isTablet(activity.getApplicationContext()); 
} 

this should be enough. tell me if this is helping. simply an activity extends from context so you even might do: UIUtils.isTablet(activity);

now another subject would be the way you are actually creating the instances which is impossible to tell from your code, Activities are started with intents and have a lifecycle, they cannot be constructed with constructors.

EDIT 2:

Your issue is in this code:

final ActivityHelper mActivityHelper = ActivityHelper.createInstance(this, context);

At this time the activity is not yet created so your context (“this”) is null.

you need to move the activity helper initialization in onCreate. the activity works a bit differently from other classes, it might be a good idea to also read a bit more on the activity lifecycle and see some more examples (try google :)).

the correct case would be:

private ActivityHelper mActivityHelper;

protected void onCreate(Bundle savedInstanceState){
super(savedInstanceState);
// here do the initialization

mActivityHelper = ActivityHelper.createInstance(this);
.
.
.


}