Android: Clear Activity Stack

Most of you are wrong. If you want to close existing activity stack regardless of what's in there and create new root, correct set of flags is the following:

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

From the doc:

public static final int FLAG_ACTIVITY_CLEAR_TASK
Added in API level 11

If set in an Intent passed to
Context.startActivity(), this flag will cause any existing task that
would be associated with the activity to be cleared before the
activity is started. That is, the activity becomes the new root of an
otherwise empty task, and any old activities are finished. This can
only be used in conjunction with FLAG_ACTIVITY_NEW_TASK.

Android: Clear the back stack

Try adding FLAG_ACTIVITY_NEW_TASK as described in the docs for FLAG_ACTIVITY_CLEAR_TOP:

This launch mode can also be used to
good effect in conjunction with
FLAG_ACTIVITY_NEW_TASK: if used to
start the root activity of a task, it
will bring any currently running
instance of that task to the
foreground, and then clear it to its
root state. This is especially useful,
for example, when launching an
activity from the notification
manager.

So your code to launch A would be:

Intent intent = new Intent(this, A.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_NEW_TASK); 
startActivity(intent);
CurrentActivity.this.finish(); // if the activity running has it's own context

// view.getContext().finish() for fragments etc.

Android: Remove all the previous activities from the back stack

The solution proposed here worked for me:

Java

Intent i = new Intent(OldActivity.this, NewActivity.class);
// set the new task and clear flags
i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
startActivity(i);

Kotlin

val i = Intent(this, NewActivity::class.java)
// set the new task and clear flags
i.flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK
startActivity(i)

However, it requires API level >= 11.

Android clear back stack between activities

If the activity being launched is already running in the current task, then instead of launching a new instance of that activity, all of the other activities on top of it will be closed and this Intent will be delivered to the (now on top) old activity as a new Intent.

Intent intent = new Intent(this, ActivityA.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);

Clear Activity Stack and start new activity in android

Use this

Intent i = new Intent(yourScreen.this,Home.class);
        i.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        i.putExtra("EXIT", true);
        startActivity(i);

and in the onCreate of the Home class, do this to check,

if (getIntent().getBooleanExtra("EXIT", false)) 
    {
        Intent i = new Intent(Home.this,Login.class);
        startActivity(i);
        finish();
    }

what this will essentially do is no matter at what activity you are, you can call the home screen with the clear top flag. In the home screen there is a check condition in the onCreate method which will help to clear the stack and take you to the login screen.. Now on the login screen,if you press back button you will exit the app as the stack is cleared..

Let me know if the problem still persists...

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

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.

how to clear the fragment stack and activity stack on a button click

You can do this way:

Clear Fragment stack:

getSupportFragmentManager().popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);

Clear Activity stack:

Intent intent = new Intent(Your_Current_Activity.this, Your_Destination_Activity.class);
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK |Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);

Hope this will help you.

Android remove Activity from back stack

FLAG_ACTIVITY_CLEAR_TOP clears your Activity stack , you can use the code below:

Intent intent = new Intent(this, Activity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);

Remember that this flag clears just Intermediate Activities , for example if you have A,B,C in your Back Stack then going from C Activity to D with this flag this does not clear Back Stack and the Stack would be A,B,C,D but if you go from Activity D to Activity A with this flag , B,C,D Activities will pop up from the stack and you will have just A in the Back Stack .

Android clear activity list on home button press

Use android:clearTaskOnLaunch="true" in your Launcher Activity in the manifest and for all other activities use android:finishOnTaskLaunch="true" in the manifest.

For a reference, have a look at the link below, this will help you doing what you want.

Kill all activities when HOME key is pressed android

Android: Clear Activity Stack