What is the lifecycle of an Android activity?

The life cycle of an Android Activity refers to the series of methods called by the system from creation to destruction. These methods can be overridden to perform different operations at various stages.

Here are the main methods of the Activity lifecycle:

1. onCreate(): called when Activity is created, used for initializing the interface and data.
2. onStart() is called when the Activity is about to become visible and is used to prepare for interaction.
3. onResume(): called when the activity becomes visible and starts to receive user focus, used for starting animations or playing music.
4. onPause(): Called when an Activity loses focus but remains visible, used to pause any ongoing operations such as saving data.
5. onStop() is called when the Activity is stopped and is completely invisible, it is used to release resources or save data.
6. onDestroy(): Called before the Activity is destroyed, used to release all resources.
7. onRestart() is called before an Activity restarts from a stopped state, to prepare for resuming interaction.

Apart from the main methods mentioned above, there are also other lifecycle methods, such as onSaveInstanceState(), which is used to save the state of an Activity so that it can be restored when recreated.

It is important to note that when a user closes an Activity by pressing the back key or performing other actions, the system will call the above methods in the normal lifecycle order. However, if the system destroys the Activity due to low memory, it will only call the onPause(), onStop(), and onDestroy() methods, and will not call the onSaveInstanceState() method. Therefore, to ensure proper saving and restoring of data, necessary data saving operations should be carried out in the onPause() or onStop() method.