What components does the HashMap data structure consist of and how does it work internally?

HashMap in Java (and Android) consists of several key components:

• Array of buckets — the main container where elements are stored. Each bucket contains either null, or a reference to a linked list or a tree (in newer versions).
• Elements (Entry or Node) — objects that store a key-value pair, as well as a reference to the next element in the chain in case of collisions.
• Hash function — computes the hash code of the key, which is then transformed into an index of the bucket array.

How it works:

1. When adding a key-value pair, the hash code of the key is calculated.
2. The hash code is transformed into an array index (usually through the operation hash & (capacity - 1)).
3. If the bucket is empty, the element is placed there.
4. If the bucket is occupied, a check of the keys in the chain is performed:
   • If the key already exists, the value is updated.
   • If not, the element is added to the chain (linked list or tree).

With a large number of collisions, the linked list can be transformed into a balanced tree to improve performance.

Example of a simplified structure:

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;
    }
}

class HashMap<K, V> {
    Node<K, V>[] table;
    int size;

    // methods put, get, etc.
}