Clear the entire history stack and start a new activity on Android

Question

Is it possible to start an activity on the stack, clearing the entire history before it?

The situation

I have an activity stack that either goes A->B->C or B->C (screen A selects the users token, but many users only have a single token).

In screen C the user may take an action which makes screen B invalid, so the application wants to take them to screen A, regardless of whether it is already in the stack. Screen A should then be the only item on the stack in my application.

Notes

There are many other similar questions, but I haven't found anything that answers this exact question. I tried calling getParent().finish() - this always results in a null pointer exception. FLAG_ACTIVITY_CLEAR_TOP only works if the activity is already on the stack.

Answer 1

In API level 11 a new Intent Flag was added just for this: Intent.FLAG_ACTIVITY_CLEAR_TASK

Just to clarify, use this:

Java

intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK); 

Kotlin

intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK 

Unfortunately for API lvl <= 10, I haven't yet found a clean solution to this. The "DontHackAndroidLikeThis" solution is indeed pure hackery. You should not do that. :)

Edit: As per @Ben Pearson's comment, for API <=10 now one can use IntentCompat class for the same. One can use IntentCompat.FLAG_ACTIVITY_CLEAR_TASK flag to clear task. So you can support pre API level 11 as well.

Answer 2

Case 1:Only two activity A and B:

Here Activity flow is A->B .On clicking backbutton from B we need to close the application then while starting Activity B from A just call finish() this will prevent android from storing Activity A in to the Backstack.eg for activity A is Loding/Splash screen of application.

Intent newIntent = new Intent(A.this, B.class); startActivity(newIntent); finish(); 

Case 2:More than two activitiy:

If there is a flow like A->B->C->D->B and on clicking back button in Activity B while coming from Activity D.In that case we should use.

Intent newIntent = new Intent(D.this,B.class); newIntent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);  newIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); startActivity(newIntent); 

Here Activity B will be started from the backstack rather than a new instance because of Intent.FLAG_ACTIVITY_CLEAR_TOP and Intent.FLAG_ACTIVITY_NEW_TASK clears the stack and makes it the top one.So when we press back button the whole application will be terminated.