In Java, can failing to override the hashCode() method when you override equals() have any performance impact when using hash based collections such as HashMap or HashSet?

Introduction / Context:
Java defines a contract between the equals() and hashCode() methods that is especially important when objects are stored in hash based collections. Many interview questions ask about correctness issues when this contract is broken. This question goes a step further and asks about performance implications, because poor hashCode() implementations can cause more collisions and slower lookups even if the program does not crash.

Given Data / Assumptions:

• We are working with Java classes that override equals() to define logical equality.
• Objects of these classes are stored in HashMap, HashSet or similar collections.
• hashCode() is not overridden, so the default implementation from Object is used.

Concept / Approach:
The general rule in Java is that if two objects are equal according to equals(), they must return the same hash code. Hash based collections such as HashMap use hashCode() to determine which bucket an entry belongs to. If equals() is overridden to consider two objects equal but hashCode() is not adjusted to return matching values, these objects may be placed in different buckets or clustered into a single bucket. This breaks expectations and can cause excessive collisions, turning average constant time operations into linear time in the worst case, which directly affects performance.

Step-by-Step Solution:
Step 1: Recall the contract that equal objects should have equal hash codes.Step 2: Recognise that hash based collections depend heavily on a well distributed hashCode() for efficient bucket lookup.Step 3: Option A states that overriding equals() without overriding hashCode() can cause many logically equal objects to collide or distribute poorly, degrading performance.Step 4: Option B incorrectly claims that hashCode() is never used by hash based collections, which contradicts their design.Step 5: Option C claims that the compiler generates an optimal hashCode(), which is not true; the default method from Object is not aware of your custom equality logic. Option D limits the impact to arrays only, so option A is the correct answer.

Verification / Alternative check:
If you write a class with a custom equals() method and fail to override hashCode(), then create many logically equal instances and insert them into a HashSet, you may observe unexpected behaviour or duplicate entries. Even when correctness appears to hold, profiling may show that operations on the set take more time because internal buckets are not used efficiently. By contrast, providing a well designed hashCode() that correlates with equals() spreads entries across buckets and maintains near constant time operations for insert and lookup on average.

Why Other Options Are Wrong:
Option B is wrong because HashMap and HashSet explicitly use hashCode() to decide where to store entries. Option C is incorrect; Java does not automatically create a hashCode() that matches your custom equals() implementation. Option D misstates the scope of the impact and ignores the fact that arrays are not hash based collections; maps and sets are the primary structures affected.

Common Pitfalls:
A common mistake is to override equals() alone for logical comparison but forget to override hashCode(). Developers may see immediate correctness issues, but sometimes the problems only appear in edge cases or as performance degradation under load. Another pitfall is writing a hashCode() that returns a constant value, which technically satisfies the equality contract but leads to the worst possible distribution in hash based collections. Best practice is always to override both equals() and hashCode() together with implementations that match the logical notion of equality for the class.

Final Answer:
Yes, if you override equals() but do not override hashCode(), many logically equal objects may end up in the same bucket, causing more collisions and degrading performance of hash based collections.