What are the Key Differences Between HashMap, LinkedHashMap, and TreeMap in Java?

Exploring the Differences between HashMap, LinkedHashMap, and TreeMap

HashMap, LinkedHashMap, and TreeMap are all Map implementations in Java, each offering unique characteristics and use cases.

HashMap is an unordered map based on a hash table. It provides constant-time operations for key-value lookup, insertion, and removal (O(1)). However, the iteration order is not guaranteed.

LinkedHashMap extends HashMap, preserving the insertion order of key-value pairs. This makes it useful when maintaining the sequence of elements is crucial.

TreeMap is a sorted map based on a Red-Black tree data structure. It maintains its key-value pairs in ascending order of keys, providing efficient sorting and searching capabilities (O(log(n))).

Comparative Analysis

The following table summarizes the key differences between these map implementations:

Feature	HashMap	LinkedHashMap	TreeMap
Iteration Order	Unordered	Insertion Order	Sorted
Get / Put / Remove / containsKey	O(1)	O(1)	O(log(n))
Interfaces	Map	Map	NavigableMap, Map, SortedMap
Null Values/Keys	Allowed	Values Only	Allowed
Fail-Fast Behavior	Not Guaranteed	Not Guaranteed	Not Guaranteed
Implementation	Buckets	Double-Linked Buckets	Red-Black Tree
Synchronization	Not synchronized	Not synchronized	Not synchronized

Hashtables: A Legacy Class

Prior to Java 1.2, Hashtables were used as the primary Map implementation. They are similar to HashMap in terms of functionality, but they are synchronized and do not allow null keys. However, they are discouraged in modern Java programming due to their performance limitations and the introduction of more efficient and convenient Map implementations.

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