singleTask Launch Mode: A -> B -> C Back Stack

What's really being asked

This question tests your fundamental understanding of Android's task and back stack management, specifically the singleTask launch mode. The interviewer is checking if you know that singleTask activities behave differently from the default standard mode, particularly how they relate to tasks. They want to see if you can trace the state of multiple tasks, not just a single back stack.

The full answer

A strong answer will walk through the process step-by-step. First, explain that Activities A and B are launched with the default standard mode. They are pushed onto the back stack of the same task. After launching B, the stack is [A, B]. Second, describe the singleTask behavior for C. The system looks for an existing task that C has an affinity for. Assuming this is the first launch of C, a new task is created. Third, state that C becomes the root Activity of this new task. The back stack for this new, foreground task is just [C]. Fourth, clarify the state of the original task. The task containing A and B is moved to the background. Its back stack remains [A, B]. Pressing the back button from C would typically navigate the user back to Activity B in the previous task.

The mistakes people make

A major red flag is confusing singleTask with other launch modes or flags. A common mistake is describing standard behavior, saying the stack simply becomes [A, B, C]. Another error is describing FLAG_ACTIVITY_CLEAR_TOP behavior without the new task context, suggesting the stack becomes [A, C] by clearing B. The defining characteristic of singleTask in this initial launch scenario is the creation of a new task. Candidates also often forget to mention that the original task [A, B] is preserved in the background.

What usually comes next

Be prepared for follow-up questions that probe deeper. "What happens if the user navigates A -> B -> C, and then launches C again from a notification?" (The existing instance of C in its own task is brought to the foreground, and its onNewIntent() method is called). "What if C had taskAffinity set to the same affinity as A and B?" (In this case, C would be launched into the existing task and clear all activities above it, resulting in a stack of [A, C]). "How does singleInstance differ from singleTask?" (singleInstance is stricter: the activity is the only activity in its task).

A concrete example

Imagine a music player app. Your main flow is MainActivity (A) -> AlbumListActivity (B). When you tap a song, it launches PlayerActivity (C), which is singleTask. You want the player to be a distinct entity. The A->B->C flow results in the PlayerActivity appearing in its own task. Pressing back from the player returns you to the AlbumListActivity in the previous task. If you later launch the player from a notification, it brings that same player instance forward instead of creating a new one.