Why getApplicationContext() in constructor of Activity throws null pointer exception?

Question

After some time spent bug hunting it turns out that :

public class MainActivity extends BaseActivity { // BaseActivity extends Activity

    public MainActivity() {
        super();
        getApplicationContext(); // NPE here
    }
}

Why ? Where is this documented ?
Froyo

Solution 1

Wait for the end of onCreate to call this method.

public class MainActivity extends BaseActivity { 

    public onCreate(Bundle savedInstanceState) {
        super(savedInstanceState);
        getApplicationContext(); //activity has a context now
    }
}

Solution 2

Just to get a feeling of what's going on. Activity extends ContextThemeWrapper which extends ContextWrapper from whom Activity inherits getApplicationContext(). ContextWrapper implements it as :

@Override
public Context  getApplicationContext() {
    return mBase.getApplicationContext(); // mBase is a Context
}

The only public constructor of ContextWrapper is :

 public  ContextWrapper(Context base) {
     mBase = base;
 } 

in ContextThemeWrapper we have :

 public  ContextThemeWrapper() {
     super(null);
 }

and since Activity does not define an explicit constructor the constructor above is called - mBase == null in Activity's constructor - boom.

_{Links from 4.2.2_r1}

Comments

[Mr_and_Mrs_D]

I found out - my problem is not that getApplicationContext(); returns null but that it throws. Where is this documented is my question - and possibly some theory on the matter :) - I believe many have fell to the lack of docs on this one - would qualify for a bug report

[Snicolas]

Check at the source code of Activity. My guess is that a context will be called and generate the NPE.

[Snicolas]

Link : grepcode.com/file/repository.grepcode.com/java/ext/…

[Snicolas]

You misunderstood the link. This is the impl inherited by Activity.

[Mr_and_Mrs_D]

return mBase.getApplicationContext(); click --> public abstract Context getApplicationContext(); :D

[Mr_and_Mrs_D]

Found some time to walk through the code - why is how, better :)

[Ben Akin]

More help, if you want to set up your buttons in this activity, you should set them up in a function outside of onCreate and then call that set up function after context = getApplicationContext();