How Does a Java HashMap Handle Hash Code Collisions?

Java HasMaps and Hash Code Collisions

Understanding the behavior of a HashMap in handling objects with identical hash codes is crucial. Your assumptions are partially correct:

• Multiple objects can have the same hash code, but it's not illegal.
• Equal objects (as determined by the equals() method) must have the same hash code.
• Non-equal objects may or may not have different hash codes.

The crux of the question lies in how the HashMap manages objects with the same hash code. Internally, the HashMap utilizes an array of buckets, each having a unique identifier derived from the hash code of its key.

When a key-value pair is added, its hash code determines the bucket it will occupy. This is done to optimize lookups by limiting the search to a specific bucket based on the hash code. However, when a hash collision occurs, the HashMap employs a collision resolution strategy.

This collision resolution strategy involves storing multiple key-value pairs in the same bucket. When a lookup occurs, the HashMap checks the hash code of the key and identifies the appropriate bucket. It then compares the input key with all the keys stored in that bucket using the equals() method to identify the matching key-value pair.

This mechanism ensures that:

• Equal objects will be stored in the same bucket, maintaining consistency.
• Non-equal objects, even with the same hash code, can still be stored and retrieved efficiently using the equals() comparison.

The hash code collision resolution strategy ensures the HashMap's efficiency while preserving key-value pair integrity, making it a powerful tool for managing key-based data structures.

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