Java Interview Question - Interval Aggregation Service Bug

Why your 2D array is actually just vibes and nulls

Context

You are working on a backend service that processes user activity windows.

Each user has multiple active sessions represented as intervals:

[startTime, endTime]

The service converts these into a 2D array for downstream analytics.

The service is failing intermittently with:

NullPointerException

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Identify the root cause of the failure

The NullPointerException happens because the code assumes that each row inside the 2D array is already allocated, but it is not.

int[][] result = new int[sessions.size()][];

This only creates the outer array, not the inner arrays.

So at this point:

result
  - null
  - null
  - null

Each result[i] is null.

when we do

result[i][0] = session[0];
result[i][1] = session[1];

This tries to access:

(null)[0]

which throws NullPointerException

Propose the fix for the bug

The root issue is that only the outer array is created, not the inner arrays. So we must ensure each row is initialized before accessing it.

Fix 1: initialize each row

Each result[i] now points to a real int[] before assignment.

Fix 2: Use rectangular allocation

If the structure is always [start, end], then:

No manual row allocation needed.

Fix 3: Use direct assignment

Since sessions.get(i) is already int[]:

int[][] result = sessions.toArray(new int[0][]);

This is a trick where we are saying “Please allocate the correct sized array for me“

if (a.length < size)
 create new array of correct size

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Happy Coding 🚀
Suraj

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