The Android app lifecycle refers to the various states and events that an app goes through during its execution. Understanding the app lifecycle is crucial for developing Android apps that behave correctly and efficiently. In this answer, we will explore the Android app lifecycle and its key methods.

1. onCreate():
The onCreate() method is the first callback method called when an app is launched or when a configuration change occurs. It is responsible for initializing the app's essential components and setting up the initial UI. This method is where you typically initialize variables, bind views, and perform other setup tasks.

2. onStart() and onStop():
The onStart() method is called when the app becomes visible to the user, either because it is launched or when it resumes from the stopped state. In contrast, the onStop() method is called when the app is no longer visible and moves to the background. In these methods, you can perform tasks like starting or stopping animations, registering or unregistering receivers, or acquiring or releasing resources.

3. onResume() and onPause():
The onResume() method is called when the app is about to start interacting with the user. It is the counterpart of onPause(), which is called when the app is partially visible and loses focus. In onResume(), you can resume animations, initialize sensors, or refresh data. In onPause(), you should release resources, save user data, or unregister any components that should not run while the app is in the background.

4. onRestart():
The onRestart() method is called when the app is being restarted after being stopped. It is followed by onStart() and onResume(). You can use this method to perform any additional setup or initialization tasks needed after the app was previously stopped.

5. onDestroy():
The onDestroy() method is called when the app is being destroyed, either because the user explicitly closed it or because the system needs to free up resources. This method allows you to release any resources held by the app, unregister receivers, or perform other cleanup tasks.

6. onSaveInstanceState() and onRestoreInstanceState():
The onSaveInstanceState() method is called before the app goes into the background or when a configuration change occurs. It provides an opportunity to save important data or UI state that should be restored when the app resumes. The saved state is passed to the method as a bundle. The onRestoreInstanceState() method is called when the app is recreated after being destroyed and allows you to restore the saved state.

7. onConfigurationChanged():
The onConfigurationChanged() method is called when a configuration change occurs, such as a screen rotation or a change in the device's locale. By overriding this method, you can handle these configuration changes manually, preventing the app from being restarted.

Understanding and correctly implementing these key methods of the app lifecycle is essential for developing robust and responsive Android apps. By utilizing these methods effectively, you can manage app state, handle configuration changes, save and restore data, and provide a seamless user experience.