Map Implementations in Java

Understanding HashMap, LinkedHashMap, TreeMap, and Hashtable

What are Map Implementations?

Map implementations in Java store key-value pairs, allowing you to quickly retrieve values using their associated keys. Each implementation differs in ordering behavior, null handling, and thread-safety. Understanding these differences helps you choose the right Map for your application.

HashMap

Simple explanation

•Stores key–value pairs
•Fast lookup
•No order guaranteed

Example

HashMapExample.java

Map<Integer, String> map = new HashMap<>();
map.put(1, "Apple");
map.put(2, "Banana");
map.put(3, "Cherry");

System.out.println(map);
// Example output (order random): {2=Banana, 1=Apple, 3=Cherry}

LinkedHashMap

Simple explanation

•Same as HashMap, but keeps the insertion order

Example

LinkedHashMapExample.java

Map<Integer, String> map = new LinkedHashMap<>();
map.put(1, "Apple");
map.put(2, "Banana");
map.put(3, "Cherry");

System.out.println(map);
// Output: {1=Apple, 2=Banana, 3=Cherry}

TreeMap

Simple explanation

•Stores keys in sorted (ascending) order
•Sorting happens automatically

Example

TreeMapExample.java

Map<Integer, String> map = new TreeMap<>();
map.put(3, "Cherry");
map.put(1, "Apple");
map.put(2, "Banana");

System.out.println(map);
// Output: {1=Apple, 2=Banana, 3=Cherry} (sorted by key)

Hashtable

Simple explanation

•Like HashMap, but thread-safe
•Legacy (old), rarely used now
•Does NOT allow null keys or values

Example

HashtableExample.java

Map<Integer, String> map = new Hashtable<>();
map.put(1, "Apple");
map.put(2, "Banana");

System.out.println(map);
// Output: {2=Banana, 1=Apple}

Quick Summary Table

Map Type	Order?	Allows null?	Thread-safe?	Notes
HashMap	No order	Yes	No	Fast, most used
LinkedHashMap	Insertion order	Yes	No	Predictable order
TreeMap	Sorted by key	No (rare cases)	No	Uses Red-Black Tree
Hashtable	No order	No	Yes	Old, replaced by ConcurrentHashMap

Key Differences at a Glance

HashMap

•Fastest

•Random order

•Allows null

LinkedHashMap

•Insertion order

•Predictable

•Allows null

TreeMap

•Auto-sorted

•By key

•No null keys

Hashtable

•Thread-safe

•Legacy

•No nulls

When to Use Each Map?

Use HashMap when:

You need fast lookups and don't care about order

Use LinkedHashMap when:

You need to maintain insertion order while still having fast access

Use TreeMap when:

You need keys automatically sorted in ascending order

Use Hashtable when:

You're working with legacy code (otherwise use ConcurrentHashMap for thread-safety)