在片段中声明 findViewById 后的 java.lang.NullpointerException

Question

I'm trying to use findViewById , but then a warning appears. I've seen serveral different ways used to resolve this warning but I don't know which is correct when it comes to my code. Does anyone know what the correct way would be to remove this warning in this context?

Method invocation 'findViewById' may produce 'java.lang.NullpointerException'

Page1Fragment.java

public class Page1Fragment extends android.support.v4.app.Fragment {

    boolean squareState;

    public Page1Fragment() {

    }

    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

        return inflater.inflate(R.layout.fragment_page1, container, false);
    }

    @Override
    public void onResume(){
        super.onResume();
        loadPreferences();
        displaySettings();
    }

    public void loadPreferences(){
        SharedPreferences pref =  this.getActivity().getSharedPreferences("settings", AppCompatActivity.MODE_PRIVATE);
        squareState = pref.getBoolean("square_state", true);
    }

    public void displaySettings() {
        if (squareState) {
            getView().findViewById(R.id.blue_square).setVisibility(View.VISIBLE);
        } else {
            getView().findViewById(R.id.blue_square).setVisibility(View.GONE);
        }
    }
}

Answer 1

I assume you're talking about a lint warning.

Depending on which part of the lifecycle the fragment is in, it may or may not have had its onCreateView method called yet.

In the case that your fragment's view hasn't yet been created (onCreateView hasn't yet been called,) calls to getView() will return null.

Thus, you'll need to check for null like so:

View contentView = getView();
if (contentView != null) {
    contentView.findViewById(R.id.blue_square).setVisibility(View.VISIBLE);
}

It's not always easy to know when a fragment is attached to an activity and its view has been created. For example, if you've started an AsyncTask to retrieve some data on a background thread and then go to update the UI, the user may have navigated off of your fragment, and getView() will return null in this case.

Have a look at the documentation for Fragment on developer.android.com for more information which details the fragment lifecycle.

Answer 2

If you know where you're calling the getView() method, (after the onCreateView and before onDestroyView ), you can ignore those warnings. This method will be returning null outside these two callbacks.

A clean way to avoid checking it over and over again would be to pass the rootView as a reference to your dislplaySettings method.

public void displaySettings(View rootView) {
    if (squareState) {
        rootView.findViewById(R.id.blue_square).setVisibility(View.VISIBLE);
    } else {
        rootView.findViewById(R.id.blue_square).setVisibility(View.GONE);
    }
}

You'll get no complains when you use it inside the onResume as follows:

@Override
public void onResume() {
    displaySettings(getView()); // You're safe here!
}

Please refer to the Fragment life cycle docs .

Bonus

To avoid some code duplication

public void displaySettings(View rootView) {
    rootView.findViewById(R.id.blue_square).setVisibility(squareState ? View.VISIBLE : View.GONE);
}