HashMap, LinkedHashMap, and TreeMap in Java: What are the Key Differences and When to Use Each?

Navigating the Nuances of HashMap, LinkedHashMap, and TreeMap in Java

Intro

As a Java developer, it's crucial to understand the distinctions between various data structures, including three prevalent options: HashMap, LinkedHashMap, and TreeMap. While they all implement the Map interface, they exhibit unique characteristics that impact their functionality and performance.

Core Differences

• Iteration Order: HashMap doesn't guarantee an iteration order, while LinkedHashMap maintains the insertion order, and TreeMap iterates based on key sorting.
• Complexity: HashMap offers O(1) complexity for get/put/remove/containsKey operations, while TreeMap operates at O(log(n)) due to its sorted nature.
• Null Values/Keys: HashMap and LinkedHashMap permit both null values and keys, but TreeMap allows only non-null values.
• Fail-Fast Behavior: The fail-fast property is not guaranteed for any of these data structures due to the potential for concurrent modification.

Underlying Implementation and Synchronization:

• HashMap utilizes a bucket mechanism for data storage, while LinkedHashMap employs double-linked buckets to preserve insertion order. TreeMap is implemented using a Red-Black Tree for sorted storage.
• None of these data structures are intrinsically synchronized, requiring explicit synchronization for concurrent access control.

Example Usage and Output:

The provided code snippet illustrates the behavior of HashMap, LinkedHashMap, and TreeMap:

// HashMap (unsorted key order)
Map<String, String> m1 = new HashMap<>();
m1.put("map", "HashMap");
m1.put("schildt", "java2");
m1.put("mathew", "Hyden");
m1.put("schildt", "java2s");
System.out.println(m1.keySet());  // [schildt, mathew, map]
System.out.println(m1.values()); // [java2s, Hyden, HashMap]

// TreeMap (sorted key order)
SortedMap<String, String> sm = new TreeMap<>();
sm.put("map", "TreeMap");
sm.put("schildt", "java2");
sm.put("mathew", "Hyden");
sm.put("schildt", "java2s");
System.out.println(sm.keySet());  // [map, mathew, schildt]
System.out.println(sm.values()); // [TreeMap, Hyden, java2s]

// LinkedHashMap (insertion order)
LinkedHashMap<String, String> lm = new LinkedHashMap<>();
lm.put("map", "LinkedHashMap");
lm.put("schildt", "java2");
lm.put("mathew", "Hyden");
lm.put("schildt", "java2s");
System.out.println(lm.keySet());  // [map, schildt, mathew]
System.out.println(lm.values()); // [LinkedHashMap, java2, Hyden]

Hashtables: A Legacy Data Structure

Before Java 1.2, Hashtables were prevalent but are now deprecated due to the more sophisticated features provided by HashMaps. Hashtables exhibit a similar behavior to HashMaps but:

• Are synchronized, which can impact performance in multithreaded environments.
• Implement the legacy Dictionary interface instead of Map.
• Require explicit type casting when accessing elements.

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