1Z0-829 Controlling Program Flow Practice Question
A developer needs to iterate over a List<String> and remove elements that are null. Which approach guarantees correct behavior without throwing a ConcurrentModificationException?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use an Iterator explicitly and call its remove() method
Iterator.remove() is the safe and recommended way to remove elements during iteration, avoiding ConcurrentModificationException. Option A is incorrect because using list.remove() inside a for-each loop modifies the list directly while iterating, causing ConcurrentModificationException. Option B is incorrect because Stream.filter() does not modify the original list; it creates a new list, which may not meet the requirement to remove elements from the original. Option C is incorrect because although a decrementing for loop may work, it is error-prone and not the guaranteed approach; it is not the idiomatic Java solution.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use a for-each loop and call list.remove() on the current element
Why it's wrong here
Modifying the list during for-each iteration throws ConcurrentModificationException.
- ✗
Use a Stream's filter() method and collect the results
Why it's wrong here
Filtering creates a new list, which is acceptable but not the only way; it doesn't modify the original list in place.
- ✗
Use a traditional for loop with an index that decrements
Why it's wrong here
While this avoids the exception, it is not the standard Java best practice for this scenario.
- ✓
Use an Iterator explicitly and call its remove() method
Why this is correct
Iterator.remove() is the safe way to remove elements during iteration.
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