Why Does My Android App Crash with 'Attempt to Invoke Virtual Method '...' on a Null Object Reference' When Navigating Between Activities?

Understanding the Error: "Attempt to Invoke Virtual Method '...' on a Null Object Reference"

When attempting to navigate from SplashActivity to LoginActivity, the application crashes with the following error:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference

Root Cause of the Problem:

The error suggests that a reference to a Java object is returning null, preventing the execution of an operation that requires a valid object. In this case, it pertains to the initialization of view elements within the LoginActivity.java code.

Solution:

The resolution lies in properly initializing the view elements before referencing them in code. The provided answer rightly identifies that initializing the view elements outside of onCreate() may lead to null references.

To fix the issue, declare the view fields in LoginActivity.java without initialization, as follows:

private EditText usernameField, passwordField;
private TextView error;
private ProgressBar progress;

Then, assign values to the fields within the onCreate() method:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);

    usernameField = (EditText)findViewById(R.id.username);
    passwordField = (EditText)findViewById(R.id.password);
    error = (TextView)findViewById(R.id.error);
    progress = (ProgressBar)findViewById(R.id.progress);
}

Additional Optimization:

As suggested in the answer, using Handler instead of Timer to schedule the transition from SplashActivity to LoginActivity is a recommended best practice, as it ensures execution on the UI thread, avoiding potential issues and ensuring a smoother user experience.

The above is the detailed content of Why Does My Android App Crash with 'Attempt to Invoke Virtual Method '...' on a Null Object Reference' When Navigating Between Activities?. For more information, please follow other related articles on the PHP Chinese website!