The answer is that the second object is not added because TreeSet considers it a duplicate based on the custom Comparable returning 0. This occurs because TreeSet relies exclusively on the compareTo() method—not the equals() method—to determine both ordering and uniqueness; when compareTo() returns zero for two objects, the set treats them as identical regardless of whether they differ in other fields. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this tests your understanding that TreeSet’s contract is built around the Comparator or Comparable logic, and a common trap is assuming equals() will save you when compareTo() says “same.” For example, if you compare Employee objects only by ID but two employees have different names, the second will be silently rejected. Remember the mnemonic: “TreeSet’s compareTo zero means ‘no room for the hero’—equals() is ignored, so consistency between compareTo and equals is key.”
1Z0-829 Working with Arrays and Collections Practice Question
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 team uses a TreeSet with a custom Comparable that returns 0 for objects that are not logically equal (e.g., based on one field but objects differ in another). What is the likely outcome when adding such objects?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The second object is not added because TreeSet considers it a duplicate
TreeSet uses the compareTo() method of the Comparable interface (or Comparator) to determine ordering and equality. When compareTo() returns 0 for two objects that are not logically equal (e.g., they share the same field used in comparison but differ in other fields), TreeSet treats them as duplicates and does not add the second object. This is because TreeSet relies on the comparison result, not the equals() method, for uniqueness.
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 second object is not added because TreeSet considers it a duplicate
Why this is correct
TreeSet uses compareTo for equality; returning 0 means duplicate.
Related concept
Read the scenario before looking for a memorised answer.
✗
The second object replaces the first
Why it's wrong here
TreeSet does not replace; it rejects duplicates.
✗
A ClassCastException is thrown
Why it's wrong here
compareTo is defined, so no ClassCastException.
✗
Both objects are added and insertion order is preserved
Why it's wrong here
TreeSet sorts, not preserves order, and duplicate detection prevents adding second.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume TreeSet uses equals() for duplicate detection, but it actually uses compareTo()/compare(), so objects that are not logically equal can be incorrectly treated as duplicates if the comparison method returns 0.
Detailed technical explanation
How to think about this question
Under the hood, TreeSet is backed by a TreeMap, which uses a Red-Black tree. When adding an element, the tree traverses nodes using compareTo(); if compareTo() returns 0, the tree considers the key already present and does not insert the new entry. This behavior can lead to subtle data loss when the Comparable implementation is inconsistent with equals(), violating the general contract of Set (which relies on equals() for uniqueness). In real-world scenarios, this often occurs when comparing by an ID field but objects differ in other fields like name or timestamp.
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 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
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.
Working with Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The second object is not added because TreeSet considers it a duplicate — TreeSet uses the compareTo() method of the Comparable interface (or Comparator) to determine ordering and equality. When compareTo() returns 0 for two objects that are not logically equal (e.g., they share the same field used in comparison but differ in other fields), TreeSet treats them as duplicates and does not add the second object. This is because TreeSet relies on the comparison result, not the equals() method, for uniqueness.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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