[java] onCreate ()에서 뷰에 액세스하는 NullPointerException

동의합니다. 사람들이 Android 개발 작업을 시작할 때 프래그먼트가 작동하는 방식을 이해하지 못하는 경우가 많기 때문에 이것은 일반적인 오류입니다. 혼란을 줄이기 위해 원래 Application is stops in android emulator 에 게시 한 간단한 예제 코드를 만들었지 만 여기에도 게시했습니다.

예는 다음과 같습니다.

public class ContainerActivity extends FragmentActivity implements ExampleFragment.Callback
{
    @Override
    public void onCreate(Bundle saveInstanceState)
    {
        super.onCreate(saveInstanceState);
        this.setContentView(R.layout.activity_container);
        if (saveInstanceState == null)
        {
             getSupportFragmentManager().beginTransaction()
                .add(R.id.activity_container_container, new ExampleFragment())
                .addToBackStack(null)
             .commit();
        }
        getSupportFragmentManager().addOnBackStackChangedListener(new OnBackStackChangedListener()
        {
            public void onBackStackChanged()
            {
                int backCount = getSupportFragmentManager().getBackStackEntryCount();
                if (backCount == 0)
                {
                    finish();
                }
            }
        });
    }

    @Override
    public void exampleFragmentCallback()
    {
        Toast.makeText(this, "Hello!", Toast.LENGTH_LONG).show();
    }
}

activity_container.xml :

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <FrameLayout
        android:id="@+id/activity_container_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>

ExampleFragment :

public class ExampleFragment extends Fragment implements View.OnClickListener
{
    public static interface Callback
    {
        void exampleFragmentCallback();
    }

    private Button btnOne;
    private Button btnTwo;
    private Button btnThree;

    private Callback callback;

    @Override
    public void onAttach(Activity activity)
    {
        super.onAttach(activity);
        try
        {
            this.callback = (Callback) activity;
        }
        catch (ClassCastException e)
        {
            Log.e(this.getClass().getSimpleName(), "Activity must implement Callback interface.", e);
            throw e;
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        View rootView = inflater.inflate(R.layout.fragment_example, container, false);

        btnOne = (Button) rootView.findViewById(R.id.example_button_one);
        btnTwo = (Button) rootView.findViewById(R.id.example_button_two);
        btnThree = (Button) rootView.findViewById(R.id.example_button_three);

        btnOne.setOnClickListener(this);
        btnTwo.setOnClickListener(this);
        btnThree.setOnClickListener(this);
        return rootView;
    }

    @Override
    public void onClick(View v)
    {
        if (btnOne == v)
        {
            Toast.makeText(getActivity(), "One.", Toast.LENGTH_LONG).show();
        }
        else if (btnTwo == v)
        {
            Toast.makeText(getActivity(), "Two.", Toast.LENGTH_LONG).show();
        }
        else if (btnThree == v)
        {
            callback.exampleFragmentCallback();
        }
    }
}

fragment_example.xml :

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

        <Button
            android:id="@+id/example_button_one"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="30dp"
            android:text="@string/hello"
            android:layout_marginLeft="20dp"
            android:layout_marginRight="20dp"/>

        <Button
            android:id="@+id/example_button_two"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/example_button_one"
            android:layout_alignRight="@+id/example_button_one"
            android:layout_below="@+id/example_button_one"
            android:layout_marginTop="30dp"
            android:text="@string/hello" />

        <Button
            android:id="@+id/example_button_three"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/example_button_two"
            android:layout_alignRight="@+id/example_button_two"
            android:layout_below="@+id/example_button_two"
            android:layout_marginTop="30dp"
            android:text="@string/hello" />

</RelativeLayout>

그리고 그것은 유효한 예제 여야합니다. Activity를 사용하여 Fragment를 표시하고 해당 Fragment에서 이벤트를 처리하는 방법을 보여줍니다. 또한 포함 된 활동과 통신하는 방법도 있습니다.

답변

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View root = inflater.inflate(R.layout.fragment_main, container,
  false);

View something = root .findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... });

return root;
}

<fragment
    android:id="@+id/myFragment"
    android:name="packagename.MainActivity$PlaceholderFragment"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" >
</fragment>