How to Resolve the Error: Attempt to Invoke Virtual Method on a Null Object Reference in Android

Why do I encounter the error 'Attempt to invoke virtual method on a null object reference' when calling Button.setOnClickListener() in my Android application?

Button myButton = findViewById(R.id.my_button);
myButton.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        // Handle button click
    }
});

The error 'Attempt to invoke virtual method on a null object reference' typically occurs in Android development when you try to call a method on an object that has not been initialized. This often happens when trying to access UI components that have not been properly linked to your code.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Button myButton = findViewById(R.id.my_button);
    myButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            // Handle button click
        }
    });
}

Causes

• The button is not properly initialized (e.g., using findViewById()) before setting the click listener.
• The ID used in findViewById() does not match any view in the current layout.
• The layout containing the button has not been set with setContentView() before trying to access the button.

Solutions

• Ensure that findViewById() is called after setContentView() in your activity or fragment.
• Check that the ID you are using in findViewById() matches the ID defined in your XML layout file.
• Make sure you are accessing the correct layout that includes the button when using findViewById().

Mistake: Calling findViewById() before setContentView() is invoked, leading to a null reference.

Solution: Always ensure that setContentView() has been called before accessing any view.

Mistake: Incorrect ID used in findViewById(), which may lead to a null object reference.

Solution: Double-check the view ID in the XML layout to ensure it matches the ID used in findViewById().

Mistake: Trying to access the button from a fragment without proper view inflation.

Solution: Ensure you are accessing the view after inflation in onCreateView() method of the fragment.