The answer is that elements are skipped when removing in a for loop because the loop's index becomes invalid after each removal. When you call remove(int index) on a list while iterating with a standard for loop that increments the index, all subsequent elements shift left by one position. This means the element that moves into the current index is never examined by the loop, as the index immediately increments past it. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of the List interface's mutability during iteration and the subtle off-by-one errors that arise from index-based removal. A common trap is assuming that a simple for loop with an incrementing index behaves like an iterator; in reality, it silently skips elements. Remember the mnemonic: "Remove shifts left, index moves right — the element in between is out of sight."
1Z0-829 Controlling Program Flow Practice Question
This 1Z0-829 practice question tests your understanding of controlling program flow. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A developer writes a method that uses a for loop to iterate over a list of strings and remove elements that match a specific pattern using the list's remove(int index) method. The developer uses an index variable that increments normally. However, after running the method, some elements that should have been removed are still present, and some elements are skipped. The list initially contains [A, B, C, D, E] and the developer expects to remove B and D. After the loop, the list is [A, C, E] as expected? Actually, the developer observes that after the loop, the list contains [A, C, D, E] (D was not removed). What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The for loop uses an index that becomes invalid after removal.
Option A is correct because when an element is removed from a list using remove(int index) while iterating with a standard for loop that increments the index variable, the indices of all subsequent elements shift left by one. This causes the loop to skip the element that moves into the current index position. In this scenario, after removing B at index 1, C shifts to index 1, but the loop increments the index to 2, so C is skipped. Then when the loop reaches index 2, it sees D (which was originally at index 3), and removes it, but the loop continues to index 3, which now holds E, so D is never processed again and remains in the list.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The for loop uses an index that becomes invalid after removal.
Why this is correct
After removal, elements shift left, so incrementing i causes skipping of the next element.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The list is unmodifiable, so remove throws UnsupportedOperationException.
Why it's wrong here
The list is modifiable as elements are removed.
✗
The remove method is called with the wrong index.
Why it's wrong here
The index is correct at the time of call, but subsequent indices shift.
✗
The for loop uses an iterator that throws ConcurrentModificationException.
Why it's wrong here
This is an index-based loop, not using an iterator.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Oracle often tests the subtlety that removing elements from a list while iterating forward with an index variable causes elements to be skipped due to index shifting, leading candidates to mistakenly think the issue is with the remove method's index or an iterator exception.
Detailed technical explanation
How to think about this question
Under the hood, ArrayList's remove(int index) method calls System.arraycopy() to shift all elements after the removed index left by one position. When using a forward-iterating for loop, the increment of the index variable after removal causes the next element to be skipped. A common fix is to iterate backward (from last index to 0) or decrement the index after removal. This behavior is specific to index-based removal and does not occur when using an Iterator's remove() method, which handles index adjustment internally.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Controlling Program Flow — This question tests Controlling Program Flow — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The for loop uses an index that becomes invalid after removal. — Option A is correct because when an element is removed from a list using remove(int index) while iterating with a standard for loop that increments the index variable, the indices of all subsequent elements shift left by one. This causes the loop to skip the element that moves into the current index position. In this scenario, after removing B at index 1, C shifts to index 1, but the loop increments the index to 2, so C is skipped. Then when the loop reaches index 2, it sees D (which was originally at index 3), and removes it, but the loop continues to index 3, which now holds E, so D is never processed again and remains in the list.
What should I do if I get this 1Z0-829 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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