HashMap, TreeMap, or LinkedHashMap: Which Java Map Should You Choose?

Comparative Analysis of HashMap, TreeMap, and LinkedHashMap in Java

Java developers often encounter the task of working with key-value pairs, and choosing the appropriate data structure for this purpose is crucial. HashMap, TreeMap, and LinkedHashMap are three widely used Map implementations that offer distinct characteristics.

HashMap: Keyless Ordering

HashMap is an unsorted Map implementation that stores key-value pairs in a hash table. It uses a hash function to distribute elements across an array of buckets, providing an average O(1) lookup time. However, the order of elements in a HashMap is not guaranteed and may change dynamically.

TreeMap: Natural Ordering

TreeMap, on the other hand, maintains its elements in sorted order according to the natural ordering of keys. It uses a Red-Black tree data structure to achieve this ordering, resulting in O(log(n)) lookup time for most operations. This is ideal for scenarios where sorted retrieval or iteration is desired.

LinkedHashMap: Insertion-Order Preservation

LinkedHashMap is an ordered Map implementation that maintains key-value pairs in the order they were inserted. It leverages double-linked buckets to achieve this, providing an O(1) lookup time while preserving the insertion order. LinkedHashMap is suitable for keeping track of the temporal relationships between keys.

Comparison Summary

The following table provides a comprehensive comparison of the three Map implementations:

Property	HashMap	TreeMap	LinkedHashMap
Iteration Order	No guaranteed order	Sorted order	Insertion order
Get / Put / Remove / ContainsKey	O(1)	O(log(n))	O(1)
Interfaces	Map	NavigableMap, Map, SortedMap	Map
Null Values/Keys	Allowed	Only values	Allowed
Fail-fast Behavior	Not guaranteed	Not guaranteed	Not guaranteed
Implementation	Buckets	Red-Black Tree	Double-linked Buckets
Is Synchronized	No	No	No

Hashtables: Deprecated Functionality

It is worth mentioning that Hashtables are a legacy class that has been deprecated in Java since version 1.2. Hashtables offer similar functionality to HashMaps but exhibit thread safety, while HashMaps are not synchronized. For most practical purposes, it is recommended to use HashMaps over Hashtables.

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