Quick Answer
The correct answer is that the value may not be retrievable using the modified key. This occurs because a HashMap determines which bucket to store a key-value pair based on the key’s hashCode at the time of insertion. When you modify a mutable key’s fields after insertion, its hashCode changes, but the entry remains in the original bucket. During retrieval, the HashMap recalculates the hash using the key’s new hashCode, which likely maps to a different bucket, so the value cannot be found—even if equals() would return true. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of how HashMap internals rely on hash code stability; it is a classic trap where candidates assume equals() alone guarantees retrieval. Remember the mnemonic: “Hash first, then equals—if the bucket changes, the key weakens.” Always use immutable keys or avoid modifying fields that affect hashCode to prevent silent data loss.
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 HashMap uses a mutable object as a key. After adding the key-value pair, the key's fields are changed such that its hashCode changes. Which statement is true?
The value can still be retrieved using the modified key
Why wrong: Hash code mismatch may prevent retrieval.
The value may not be retrievable using the modified key
Changed hash leads to lookup in wrong bucket.
The entry is automatically moved to the correct bucket
Why wrong: HashMap does not monitor key mutations.
An IllegalArgumentException is thrown when modifying the key
Why wrong: No exception from key mutation.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The value may not be retrievable using the modified key
When a HashMap uses a mutable object as a key and the key's hashCode changes after insertion, the key is stored in the bucket corresponding to its original hash code. The modified key's new hash code will likely map to a different bucket during retrieval, so the value may not be found. Option B is correct because the value may not be retrievable using the modified key, depending on whether the new hash code leads to the same bucket and whether equals() still matches.
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.
The value can still be retrieved using the modified key
Why it's wrong here
Hash code mismatch may prevent retrieval.
The value may not be retrievable using the modified key
Why this is correct
Changed hash leads to lookup in wrong bucket.
Related concept
Read the scenario before looking for a memorised answer.
The entry is automatically moved to the correct bucket
Why it's wrong here
HashMap does not monitor key mutations.
An IllegalArgumentException is thrown when modifying the key
Why it's wrong here
No exception from key mutation.
Common exam traps
The trap here is that candidates assume HashMap automatically handles key mutations or that Java enforces key immutability, but in reality, the contract requires keys to be immutable for correct behavior, and no runtime check is performed.
Detailed technical explanation
Under the hood, HashMap stores entries in an array of buckets, using the key's hashCode() to determine the bucket index and equals() to locate the exact entry within that bucket. If a key's hashCode changes after insertion, the entry remains in the original bucket, and a subsequent get() using the modified key will compute a different bucket index, likely missing the entry. This is a common pitfall in production systems where mutable objects like java.util.Date or custom objects with mutable fields are used as keys, leading to subtle memory leaks or data loss.
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: The value may not be retrievable using the modified key — When a HashMap uses a mutable object as a key and the key's hashCode changes after insertion, the key is stored in the bucket corresponding to its original hash code. The modified key's new hash code will likely map to a different bucket during retrieval, so the value may not be found. Option B is correct because the value may not be retrievable using the modified key, depending on whether the new hash code leads to the same bucket and whether equals() still matches.
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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