Java class that implements Map and keeps insertion order?

java

map-implementation

linkedhashmap

performance-optimization

byAlex Kataev·Jan 5, 2025

For a Map maintaining insertion order, go with LinkedHashMap. It's as simple as:

Map<KeyType, ValueType> map = new LinkedHashMap<>();
map.put(key1, value1); // Like adding a favorite song to your playlist
map.put(key2, value2); // And another one...
// Plays songs in the order you added
map.forEach((key, value) -> System.out.println("Now playing " + key + ": " + value));

Voila! key1: value1, key2: value2 appear in the order added. Ideal for ordered Map scenarios.

Why find love with LinkedHashMap?

The LinkedHashMap preserves the order of entry, singing a symphony of performance with O(1) time complexity for put, get, containsKey, and remove operations. Allowing one null key and multitudes of null values, it strums a chord of versatility.

Picking your Map: Quick Glance at Alternatives

Eyes set on sorting the keys in natural or customized order? The TreeMap calls you. Though it shakes a leg with O(log n) performance, it's the tune to hum when sorted key order is your groove.

For some API Jazz, the SortedMap or NavigableMap interfaces are your saxophones. Amplifying flexible code creation, they keep the ordered or navigable map characteristics alive.

If immutable rhythm is your jam, consider Guava library's ImmutableMap. It offers a once set, sealed forever philosophy, without put or remove operations.

Essential notes for using LinkedHashMap

Striking the right notes when playing with a LinkedHashMap:

Iterating over keySet(), entrySet(), or values() pulls the curtain to elements in insertion order.
In access-order mode, the iteration order arranges keys from least-recently to most-recently accessed. Scala Elasticsearch anyone?
When using custom object keys, put aside your coffee, override .equals() and .hashCode() to avoid tripping over unexpected behavior.

Zooming in: Detailed Look at Ordered Map Choices

Performance: How quickly does your code dance?

Though LinkedHashMap fires up with O(1) complexity, the TreeMap might strike a slowdown note with O(log n) complexity. Gauge your map's size and access frequency before choosing the rhythm.

API: Your coding canvas

Incorporating interfaces like SortedMap or NavigableMap elevates your API to a versatile masterpiece, allowing easy scenario adaptation without major recoding.

Immutable Map: The disciplined dancer

Eyes on an immutable map with preserved insertion order? Guava's ImmutableMap marches in. Mind the absence of put or remove methods; once choreographed, it doesn’t miss a beat.

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