LinkedHashMap Maintains Insertion Order

In Java, LinkedHashMap is a Map class implemented based on a hash table and a doubly linked list. It inherits from the HashMap class and can preserve the insertion order of elements. Elements stored in LinkedHashMap are kept in key-value pair format, where both keys and values can be null.

The implementation principle of LinkedHashMap is to maintain the insertion order of elements using a doubly linked list on top of HashMap. Each element contains pointers to its predecessor node and successor node, thereby preserving the insertion order. When an element is added to a LinkedHashMap, it is first inserted into the hash table of HashMap, and then inserted into the doubly linked list according to the insertion order.

The main methods of LinkedHashMap include:

• put(K key, V value): Adds the specified key-value pair to the LinkedHashMap.
• remove(Object key): Removes the key-value pair corresponding to a specified key from the LinkedHashMap.
• get(Object key): Retrieves the value corresponding to a specified key.
• clear(): Removes all key-value pairs in the LinkedHashMap.
• size(): Returns the number of key-value pairs in the LinkedHashMap.
• keySet(): Returns a set view of all keys contained in this map.
• values(): Returns a collection view of all values contained in this map.
• entrySet(): Returns a set view of all entries (keys and values) contained in this map.

It is important to note that the order of elements in LinkedHashMap is maintained based on their insertion order. Therefore, when iterating over a LinkedHashMap, the sequence of elements follows the insertion order rather than any relationship between keys or values such as size comparison.

The advantages of LinkedHashMap are its ability to perform element addition, deletion, and lookup operations quickly while preserving the insertion order. However, its disadvantage is that it has higher space overhead compared to HashMap because it needs to maintain a doubly linked list. Additionally, since LinkedHashMap is implemented based on a hash table, the time complexity for its lookup and delete operations is typically O(1). In extreme cases, however, it may degrade to an O(n) linear search; therefore, one must pay attention to the usage scenarios and performance of LinkedHashMap. Generally speaking, if preserving element insertion order is required and space overhead requirements are not stringent, using a LinkedHashMap can be considered.