Understanding the Underlying Mechanism of Java HashMap

When a programmer leaves a job, removing code comments may not be illegal but can breach internal policies and professional ethics.

In Java, HashMap is a fundamental data structure implemented using a hash table. Keys are hashed to determine an index in an internal array (bucket). Collisions are resolved with linked lists, and since Java 8, long chains are transformed into red‑black trees to improve lookup from O(n) to O(log n).

The following simplified implementation demonstrates the core logic of put, hash calculation, bucket indexing, and collision handling:

public class CustomHashMap<K, V> 
{
    private static final int DEFAULT_CAPACITY = 16;
    private Node<K, V>[] table;

    public CustomHashMap()
    {
        table = new Node[DEFAULT_CAPACITY];
    }

    static class Node<K, V> 
    {
        final int hash;
        final K key;
        V value;
        Node<K, V> next;

        Node(int hash, K key, V value, Node<K, V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }
    }

    public void put(K key, V value)
    {
        int hash = key.hashCode();
        int index = hash & (table.length - 1);
        Node<K, V> newNode = new Node<>(hash, key, value, null);

        if (table[index] == null) {
            table[index] = newNode;
        } else {
            Node<K, V> current = table[index];
            while (current.next != null) {
                if (current.key.equals(key)) {
                    current.value = value;
                    return;
                }
                current = current.next;
            }
            current.next = newNode;
        }
    }
}

Real HashMap also supports resizing when the load factor (default 0.75) is exceeded, which rehashes entries into a larger array. In multithreaded scenarios, concurrent modifications can cause loops, so ConcurrentHashMap is recommended.

Understanding these details helps answer interview questions such as “Explain HashMap’s underlying principle” and prepares candidates for deeper source‑code analysis.