Explain the concept of a linked hash map in data structures.

A linked hash map is a data structure that combines the features of a hash table and a linked list.

In more detail, a linked hash map is a hash table with a doubly linked list running through it. This means it maintains a set of entries, each of which contains a key and a value. The entries are organised into a hash table, which allows for efficient retrieval of an entry based on its key. The entries are also linked together in the order in which they were inserted into the map. This linked list determines the iteration ordering of the map, which is typically the order in which keys were inserted (insertion-order).

The hash table part of a linked hash map provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. The linked list part of the data structure allows for iteration over the map's contents in the order of insertion. This can be useful in situations where you need to iterate over the elements of a map in a specific order, such as when you're implementing a least recently used (LRU) cache.

In a linked hash map, each entry in the map is a node in the linked list. When an entry is accessed, it is moved to the end of the list. When the map exceeds its capacity, the entry at the beginning of the list (which is the least recently accessed entry) is removed. This makes linked hash maps an excellent choice for implementing caches.

In Java, for example, the LinkedHashMap class is part of the Java Collections Framework. It extends the HashMap class and implements the Map interface. It has methods for adding entries, removing entries, and accessing entries. It also has methods for iterating over the entries in the order they were inserted or in the order they were last accessed.

In summary, a linked hash map is a versatile data structure that combines the efficiency of a hash table with the ordered access of a linked list. It is particularly useful in situations where you need to maintain a collection of key-value pairs and iterate over them in a specific order.