How to Resolve NullPointerException When Accessing Views in onCreate() of an Android Activity

What causes NullPointerException when using findViewById() in the onCreate() method of an Android activity?

// Example code where NPE occurs
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    View something = findViewById(R.id.something); // Possible NullPointerException
    something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }
}

The NullPointerException (NPE) in this scenario is often caused by calling findViewById() on views that are set in a layout that is not currently inflated. In the provided context, this happens because the view 'something' is located in the layout defined for the PlaceholderFragment, not in the activity's layout.

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.fragment_main, container, false);

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

    return view;
}

Causes

• The ID used in findViewById() does not exist in the current layout.
• The view is being accessed before it has been instantiated or inflated.
• Incorrectly referencing the fragment layout instead of the activity layout.

Solutions

• Ensure that the ID used in findViewById() matches a view in the currently set content layout.
• Move the view reference and initialization code to the fragment's onCreateView() method where the layout is inflated.
• Use getView() from fragment to access views once the fragment's layout is attached.

Mistake: Accessing views outside of the fragment's lifecycle methods.

Solution: Always perform view lookups in onCreateView() of the fragment.

Mistake: Assuming all views are accessible in the activity without checking the appropriate layout.

Solution: Verify that the views are part of the inflated layout in use.