← All writing

Java

Top 10 Java HashMap Interview Questions

Interview questions on HashMap internals, hashing, collisions, performance, and concurrency.

2 min read
1 view

  • What is a HashMap in Java?

    • A HashMap is a part of Java's Collections Framework that stores key-value pairs, allowing fast retrieval using keys. It implements the Map interface and uses a hash table for storage.

  • How does HashMap work internally?

    • HashMap uses a hash table with an array of buckets. Keys are hashed using hashCode(), and the resulting index determines the bucket. Collisions are resolved using linked lists or, in Java 8+, balanced trees for better performance.

  • What is the time complexity of HashMap operations?

    • Get and put operations are O(1) on average, assuming a good hash function. In worst-case scenarios (many collisions), it can degrade to O(n) for linked lists or O(log n) for trees (Java 8+).

  • What happens when two keys have the same hash code in a HashMap?

    • If two keys have the same hash code, they are placed in the same bucket. Collisions are resolved by storing entries in a linked list or tree, and equals() is used to differentiate keys.

  • Is HashMap thread-safe?

    • No, HashMap is not thread-safe. For thread-safe operations, use ConcurrentHashMap or wrap HashMap with Collections.synchronizedMap().

  • What is the difference between HashMap and ConcurrentHashMap?

    • HashMap is not thread-safe and allows null keys/values. ConcurrentHashMap is thread-safe, supports concurrent access, and does not allow null keys/values.

  • Can HashMap store null keys or values?

    • Yes, HashMap allows one null key and multiple null values.

  • What is the load factor and initial capacity in HashMap?

    • Initial capacity is the number of buckets (default 16). Load factor is the threshold (default 0.75) at which the HashMap resizes (doubles) when the number of entries exceeds capacity * load factor.

  • How does HashMap handle resizing?

    • When the number of entries exceeds the load factor threshold, HashMap doubles its bucket array size and rehashes all entries to redistribute them into new buckets.

  • What is the importance of hashCode() and equals() in HashMap?

    • hashCode() determines the bucket where the key is stored. equals() is used to compare keys within the same bucket to handle collisions and ensure accurate key retrieval.

Also see.

More articles →