Krabi, Thailand

How HashMap internally works in Java 8?

1. Structure of a HashMap

Bucket Array:

• The primary data structure is an array of buckets, where each bucket can hold multiple key-value pairs (nodes). The position of these buckets is determined by hashing the keys.

A bucket can be:

• Empty
• Contain a single key-value pair
• Contain multiple key-value pairs (collision), either as a Linked List or a Red-Black Tree.

Node Structure:

Each key-value pair is represented as a Node<K,V> object, defined internally like this:

static class Node<K, V> {
    final int hash;  // Precomputed hash code of the key
    final K key;     // The key object
    V value;         // The value associated with the key
    Node<K, V> next; // Reference to the next node in the same bucket
}

static class Entry<K,V> implements Map.Entry<K,V> 
{
        final K key;
        V value;
        Entry<K,V> next;
        int hash;
}

As you see, this inner class has four fields. key, value, next and hash.