How HashMap Works?

Short Answer:
HashMap stores key-value pairs using hashing. It uses hashCode() to find the bucket and equals() to resolve collisions.

Detailed Explanation:
HashMap internally uses an array of buckets (Node[] table). Each bucket stores entries as LinkedList (Java 7) or LinkedList + Red-Black Tree (Java 8+).

Internal Working Steps

1. hashCode() is calculated for the key.
2. Hash is converted into index → (n-1) & hash.
3. If bucket empty → store entry.
4. If collision → use equals() to check duplicate.
5. If bucket size > 8 → convert to Red-Black Tree (Java 8).

equals() and hashCode() Contract

Short: If two objects are equal, their hashCode must be equal.

• If equals() returns true → hashCode() must be same.
• If hashCode same → equals() may or may not be true.
• Always override both together.

@Override
public int hashCode() {
    return id;
}

@Override
public boolean equals(Object obj) {
    if(this == obj) return true;
    if(!(obj instanceof Student)) return false;
    Student s = (Student) obj;
    return this.id == s.id;
}

How Resizing Works

Default capacity = 16 Default load factor = 0.75 Resize happens when:

Size > Capacity × Load Factor

Example: 16 × 0.75 = 12 When 13th element inserted → capacity doubles to 32.

• New array created.
• All entries rehashed.
• Time-consuming operation.

ConcurrentHashMap Internal Working

Short: Thread-safe version of HashMap without full locking.

Java 7 → Segmented locking Java 8+ → CAS (Compare and Swap) + synchronized on bucket level

• No null key allowed.
• Better performance in multi-threaded apps.
• Reads are mostly non-blocking.

HashMap vs Hashtable vs ConcurrentHashMap

Important Interview Points

• Time Complexity → O(1) average.
• Worst case → O(n).
• Treeify threshold = 8.
• Untreeify threshold = 6.
• HashMap not synchronized.