Quick Answer
The answer is false. This is correct because a TreeSet, as an implementation of the Set interface, enforces uniqueness by design—when you call add("Bob") on a set that already contains "Bob", the add method returns false to indicate the element was not inserted, and the set remains unchanged. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this tests your understanding of the Set contract and the specific behavior of TreeSet, which also maintains elements in sorted order. A common trap is confusing TreeSet with List or forgetting that add returns a boolean; many candidates assume the operation silently fails or throws an exception, but the return value is the key. Memory tip: think "Set says 'no duplicates'—add returns false when it's déjà vu."
This 1Z0-829 practice question tests your understanding of working with arrays and collections. 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 TreeSet<String> is used to store a list of employee names. The set currently contains "Alice", "Bob", "Charlie". What is the output after calling set.add("Bob")?
Exception
Why wrong: Adding a duplicate does not throw an exception; it simply returns false.
false
TreeSet implements Set, which does not allow duplicates; add returns false.
The set becomes ["Alice","Bob","Bob","Charlie"]
Why wrong: Sets cannot contain duplicate elements.
true
Why wrong: false is returned because "Bob" already exists.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
false
Option B is correct because a TreeSet implements the Set interface, which does not allow duplicate elements. When calling add("Bob") on a TreeSet that already contains "Bob", the add method returns false, indicating that the element was not added, and the set remains unchanged.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Exception
Why it's wrong here
Adding a duplicate does not throw an exception; it simply returns false.
false
Why this is correct
TreeSet implements Set, which does not allow duplicates; add returns false.
Related concept
Read the scenario before looking for a memorised answer.
The set becomes ["Alice","Bob","Bob","Charlie"]
Why it's wrong here
Sets cannot contain duplicate elements.
true
Why it's wrong here
false is returned because "Bob" already exists.
Common exam traps
The trap here is that candidates may confuse the return value of add() with the behavior of a List, expecting an exception or a successful insertion, but the Set interface mandates that duplicates are silently rejected by returning false.
Detailed technical explanation
Under the hood, TreeSet is backed by a TreeMap, where the added element is stored as the key and a constant dummy object as the value. The add method calls TreeMap.put(), which returns the previous value if the key already exists, causing TreeSet.add() to return false. This behavior is consistent with the Set contract defined in the Java Collections Framework, ensuring uniqueness via the element's compareTo() method (or Comparator) rather than equals().
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 1Z0-829 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Working with Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
The correct answer is: false — Option B is correct because a TreeSet implements the Set interface, which does not allow duplicate elements. When calling add("Bob") on a TreeSet that already contains "Bob", the add method returns false, indicating that the element was not added, and the set remains unchanged.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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