CODEWITHSUNDEEP
javaandroidandroid-fragments
Gabi
Gabi

Reputation: 667

Getting a value from parent activity returns null

I have an activity and a fragment working with. In the activity, i'm initializing the current user like this(manually):

User currentUser;
@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        currentUser = new User("email", "name", "secondName", "age");
\\.....}

the MainActivity contains a method like this :

public User getCurrentUser() {
        return currentUser;
    }

now, here is the problem : in the child fragment i'm calling getCurrentUser from mainActivity like this :

User u = MainActivity.newInstance().getCurrentUser();

MainActivity.newInstance() - defined in mainActivity like this, returning a static instance of activity :

private static MainActivity mainActivityInstance = new MainActivity();

    public static MainActivity newInstance() {
        return mainActivityInstance;
    }

the problem is that getCurrentUser returns a null object reference even though currentUser is initialized in mainActivity with valid data. So, calling for example 

u.getName();

will throw me a NullPointerException How can i solve this? i feel like i'm freaking out Thanks.

Upvotes: 0

Views: 766

Answers (4)

Leandro Ocampo
Leandro Ocampo

Reputation: 2004

As many people said: "You can't instantiate an Activity in a Fragment".

Second: there are many options you can implement in order to get the User in a Fragment. Here are the common options:

1 - Pass the user as a parameter in the fragment:

public static FragmentChild newInstance(User user){
  Bundle b = new Bundle();
  b.putSerializable("USER_PARAM",user);
  FragmentChild fragment = new FragmentChild();
  fragment.putArguments(b);
  return fragment;
}

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    User user = getArguments().getSerializable("USER_PARAM")
}

public class User implements Serializable {}

2 - Use the method ((MainActivity)getActivity()).getCurrentUser(). It will work but if you want to do it in the right way you should use an interface instead of using the MainActivity directly.

Interface option:

public class MainActivity extends AppCompatActivity implements UserDeliver {

@Override
public User getUser(){
   return user;
}


}

public class MyFragment extends Fragment {

  private UserDeliver userDeliver;      
  private User user;    

  @Override
  public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    user = userDeliver.getUser();
  }

  @Override
  public void onAttach(Context context) {
    super.onAttach(context);
    userDeliver = (UserDeliver) context;
  }

  @Override
  public void onDetach() {
   super.onDetach();
   userDeliver = null;
  }

  public interface UserDeliver {
    User getUser();
  }
}

Upvotes: 3

Gabe Sechan
Gabe Sechan

Reputation: 93561

First off- you can't create an Activity via new. It will compile, but not be properly initialized because only the framework can do that.

Secondly- never hold an activity in a static variable. This will leak memory. Lots of memory, because the entire view hierarchy has a reference from the Activity. You will cause OOM errors doing this.

Upvotes: 1

CommonsWare
CommonsWare

Reputation: 1006604

MainActivity.newInstance() - defined in mainActivity like this, returning a static instance of activity :

Never create an instance of an activity directly yourself.

the problem is that getCurrentUser returns a null object reference even though currentUser is initialized in mainActivity with valid data

Those activity instances are separate Java objects.

From a fragment, call getActivity() to retrieve the activity object that is hosting the fragment. This is covered in any good book or course on Android app development.

Upvotes: 4

Alvaro
Alvaro

Reputation: 511

U are generating a newInstance of the MainActivity everytime you do MainActivity.newInstance()

Try to retrieve your user with ((MainActivity)getActivity).getCurrentUser();

Or create the instance of your user in the fragment

Upvotes: 0

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