Android Interview Questions & Answer
The onCreate() method is called once during the Activity lifecycle, either when the application starts, or when the Activity has been destroyed and then recreated, for example during a configuration change.
The onStart() method is called whenever the Activity becomes visible to the user, typically after onCreate() or onRestart().
If finish() is called in the OnCreate method of an activity, the system will invoke onDestroy() method directly.
As onCreate() of an Activity is called only once, this is the point where most initialization should go. It is inefficient to set the content in onResume() or onStart() (which are called multiple times) as the setContentView() is a heavy operation.
OnRestoreInstanceState() - When activity is recreated after it was previously destroyed, we can recover the saved state from the Bundle that the system passes to the activity. Both the onCreate() and onRestoreInstanceState() callback methods receive the same Bundle that contains the instance state information. But because the onCreate() method is called whether the system is creating a new instance of your activity or recreating a previous one, you must check whether the state Bundle is null before you attempt to read it. If it is null, then the system is creating a new instance of the activity, instead of restoring a previous one that was destroyed.
onSaveInstanceState() - is a method used to store data before pausing the activity.
FLAG_ACTIVITY_CLEAR_TASK is used to clear all the activities from the task including any existing instances of the class invoked. The Activity launched by intent becomes the new root of the otherwise empty task list. This flag has to be used in conjunction with FLAG_ ACTIVITY_NEW_TASK.
FLAG_ACTIVITY_CLEAR_TOP on the other hand, if set and if an old instance of this Activity exists in the task list then barring that all the other activities are removed and that old activity becomes the root of the task list. Else if there’s no instance of that activity then a new instance of it is made the root of the task list. Using FLAG_ACTIVITY_NEW_TASK in conjunction is a good practice, though not necessary.
A ContentProvider provides data from one application to another, when requested. It manages access to a structured set of data. It provides mechanisms for defining data security. ContentProvider is the standard interface that connects data in one process with code running in another process.
When you want to access data in a ContentProvider, you must instead use the ContentResolver object in your application’s Context to communicate with the provider as a client. The provider object receives data requests from clients, performs the requested action, and returns the results.
Start by making sure your Android application has the necessary read access permissions. Then, get access to the ContentResolver object by calling getContentResolver() on the Context object, and retrieving the data by constructing a query using ContentResolver.query().
The ContentResolver.query() method returns a Cursor, so you can retrieve data from each column using Cursor methods.
Standard: It creates a new instance of an activity in the task from which it was started. Multiple instances of the activity can be created and multiple instances can be added to the same or different tasks.
Example: Suppose there is an activity stack of A -> B -> C. Now if we launch B again with the launch mode as “standard”, the new stack will be A -> B -> C -> B.
SingleTop: It is the same as the standard, except if there is a previous instance of the activity that exists in the top of the stack, then it will not create a new instance but rather send the intent to the existing instance of the activity.
Example: Suppose there is an activity stack of A -> B. Now if we launch C with the launch mode as “singleTop”, the new stack will be A -> B -> C as usual.
Now if there is an activity stack of A -> B -> C. If we launch C again with the launch mode as “singleTop”, the new stack will still be A -> B -> C.
SingleTask: A new task will always be created and a new instance will be pushed to the task as the root one. So if the activity is already in the task, the intent will be redirected to onNewIntent() else a new instance will be created. At a time only one instance of activity will exist.
Example: Suppose there is an activity stack of A -> B -> C -> D. Now if we launch D with the launch mode as “singleTask”, the new stack will be A -> B -> C -> D as usual.
Now if there is an activity stack of A -> B -> C -> D. If we launch activity B again with the launch mode as “singleTask”, the new activity stack will be A -> B. Activities C and D will be destroyed.
SingleInstance: Same as single task but the system does not launch any activities in the same task as this activity. If new activities are launched, they are done so in a separate task.
Example: Suppose there is an activity stack of A -> B -> C -> D. If we launch activity B again with the launch mode as “singleTask”, the new activity stack will be:
Task1 - A -> B -> C and Task2 - D
When the screen is rotated, the current instance of activity is destroyed a new instance of the Activity is created in the new orientation. The onRestart() method is invoked first when a screen is rotated. The other lifecycle methods get invoked in the similar flow as they were when the activity was first created.
The most common approach these days would be to use a combination of ViewModels and onSaveInstanceState(). So how we do we that?
Basics of ViewModel: A ViewModel is LifeCycle-Aware. In other words, a ViewModel will not be destroyed if its owner is destroyed for a configuration change (e.g. rotation). The new instance of the owner will just re-connected to the existing ViewModel. So if you rotate an Activity three times, you have just created three different Activity instances, but you only have one ViewModel.
So the common practice is to store data in the ViewModel class (since it persists data during configuration changes) and use OnSaveInstanceState to store small amounts of UI data.
For instance, let’s say we have a search screen and the user has entered a query in the Edittext. This results in a list of items being displayed in the RecyclerView. Now if the screen is rotated, the ideal way to prevent resetting of data would be to store the list of search items in the ViewModel and the query text user has entered in the OnSaveInstanceState method of the activity.