How is the internal structure of LinkedHashMap organized? What is an element of the doubly linked list?

The internal structure of LinkedHashMap in Java combines a hash table and a doubly linked list to preserve the order of element insertion.

• The core is an array of buckets (hash table), where each bucket contains a chain of elements with the same hash.
• Each element (Entry or Node) contains a key, a value, a reference to the next element in the chain (to resolve collisions), and two additional references — to the previous and next elements in insertion order.

Thus, an element of the doubly linked list is an Entry object, which holds before and after links pointing to neighboring elements in insertion order. This allows LinkedHashMap to maintain iteration order based on insertion, while also providing quick access by key through the hash table.

A simplified example of an element's structure:

class Entry<K,V> {
    final K key;
    V value;
    Entry<K,V> next;      // for chain in bucket
    Entry<K,V> before;    // previous in insertion order
    Entry<K,V> after;     // next in insertion order
}