Describe the back stack for A -> B -> C with singleTask
Tests understanding of the singleTask launch mode. A great answer explains that launching C destroys B, as singleTask clears all activities above it in the task. The final back stack becomes [A, C]. A red flag is confusing this with singleTop.
What's really being asked
This question evaluates your precise knowledge of Android's launchMode attribute, specifically the behavior of singleTask. Interviewers want to see if you understand how different launch modes manipulate the task's back stack, which is crucial for building predictable and user-friendly navigation. It separates candidates who have a surface-level understanding from those who grasp the underlying mechanics of the Android framework.
The full answer
A strong answer walks through the process step-by-step. First, state the initial stack after navigating A -> B is [A, B]. Second, explain that when Activity C is launched with launchMode="singleTask", the system checks the current task. Assuming default task affinity, it creates a new instance of C. Third, and most importantly, it invokes the "clear top" behavior. This means any activities in the task that are above the new instance are destroyed. In this scenario, Activity B is destroyed. Fourth, state the final back stack is [A, C].
The mistakes people make
A frequent red flag is confusing singleTask with other launch modes. A candidate might say the stack becomes [A, B, C], which is the behavior for standard or singleTop in this scenario. Another common error is describing singleInstance behavior, suggesting C is launched in a completely new, separate task. This only happens with singleInstance or if C's taskAffinity is explicitly set to be different. A vague answer that just says "C is brought to the front" without explaining what happens to B is also a weak signal.
What usually comes next
Be prepared for follow-up questions like: "What happens if the user navigates A -> B -> C -> B and then to C again?" (The second launch of C would call onNewIntent() on the existing C instance and destroy B, leaving the stack as [A, C]). Another is: "How would the behavior change if C was singleInstance?" (C would be created in its own new task, leaving the original task's stack as [A, B]. Pressing back from C would return to B). "What if C had a different taskAffinity?" (It would create a new task with C at the root).
A concrete example
In the initial state, the task's back stack is [A, B]. When the intent to launch Activity C is fired, the Android system processes its singleTask launch mode. It creates a new instance of C and places it on the stack. It then clears all activities above it. Since B was on top of A, B is destroyed. The final back stack contains only [A, C]. If the user presses the back button from Activity C, they will navigate back to Activity A, as B is no longer in the history.
Interview question
An app navigates from Activity A to B. It then launches Activity C, which has `launchMode="singleTask"`. What is the final state of the task's back stack?
- a.[C]
- b.[A, B, C]
- c.[A, C]Correct
- d.[A, B] in one task, and [C] in a new task
Why? this is the answer
Launching an activity with `singleTask` clears all activities on top of it in the current task. Thus, Activity B is destroyed, leaving the stack as [A, C]. The stack would be [A, B, C] only if C had the default `standard` launch mode.
Just read this? Test yourself on what you have been reading.
Read the original → developer.android.com
- #android
- #activity
- #launchmode
- #back stack
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