Java Ordered Map

java

map-implementation

java-data-structures

performance-optimization

byNikita Barsukov·Feb 26, 2025

A quick way to create an ordered map in Java is by using the LinkedHashMap class, which maintains the insertion-order via its doubly-linked list across all entries.

Here's a handy example:

Map<String, Integer> orderedMap = new LinkedHashMap<>();
orderedMap.put("One", 1); // Step 1: Learn to count
orderedMap.put("Two", 2); // Step 2: Continue counting
orderedMap.put("Three", 3); // Step 3: Become a pro at counting

orderedMap.forEach((key, value) -> System.out.println(key + " = " + value));

And voila, the map echoes your keys in the order you whispered them:

One = 1
Two = 2
Three = 3

The conclusion here: if key insertion order preservation gives you happiness, LinkedHashMap is your new best friend.

Understand your Map options

Maps in Java are like clothes, not one-size-fits-all. So let's play a quick game of "Map or Not":

LinkedHashMap: Remembers the key insertion order. Not here for a long time, but here for a good time.
TreeMap: Sorts entries based on key values according to their natural ordering or a custom Comparator. The nerdy map you never knew you needed.
ConcurrentSkipListMap: Thread-safe and sorted. Perfect for when two threads want to dance—but not step on each other.
EnumMap: Key order is the enum declaration order. Enum-erate on that!

Be sure to consider the time complexity of your operations. TreeMap operates in logarithmic time for common operations, while LinkedHashMap pulls it off in constant time for put and remove operations.

Let's Explore more Map Features

Did you know maps in Java are also feature-packed? Here are some Bob-the-Builders:

NavigableMap: Traverse through the keys like a GPS sifting through its routes.
SortedMap: When the accountant within you gets fussy about the order.

When your map requirements aren't too picky, and performance is key, fall back on the reliable anarchy of HashMap.

Look Outside — Explore Third-party Libraries

Java's not the only game in town! Some third-party libraries provide additional convenience:

Google Guava: Has a BiMap (a bat with two wings). Enables quick switching between keys and values.
Eclipse Collections: Focused on memory and performance optimization. For those who enjoy tweaking the radio to get the perfect sound.

Navigating the world of Maps

Align with Requirements

Examine your requirements before picking your tactical map. Your artillery includes:

Insertion order or sorted keys?
Frequent insertions/deletions or read-mostly?
Thread-safe without explicit synchronization?

Tie Performance with Use-case

Here's a gentle reminder: TreeMap offers O(log n) operations due to its red-black tree handling while LinkedHashMap offers O(1) operations but at a cost of extra memory.