Is the order guaranteed for the return of keys and values from a LinkedHashMap object?

java

linked-list

hash-table

map-implementation

byAlex Kataev·Jan 21, 2025

Undoubtedly, LinkedHashMap preserves insertion order. Meaning, when iterating over its entries, keys, or values, they will reflect the order they were added in:

LinkedHashMap<Integer, String> map = new LinkedHashMap<>();
map.put(1, "A"); //Knock, Knock! Who's there? A!
map.put(2, "B"); //B who? B prepared for humor in code comments!
map.put(3, "C"); //C who? C how easy traversing LinkedHashMap is!

map.forEach((key, value) -> System.out.println(key + ":" + value)); //Every time, it's party time: 1:A 2:B 3:C

You can always count on 1:A 2:B 3:C, a feature exclusive to LinkedHashMap.

The anatomy of LinkedHashMap: Under the hood

To appreciate LinkedHashMap, let's first dissect its structure. It brews a unique combination brewing a hash table for rapid access and a doubly-linked list to uphold the element order. This particular design mechanism ensures the order of return of keySet(), values(), and entrySet() coinciding with the order of their insertion.

Feeding your curiosity: KeySet and Values

When scrutinizing the predictability of LinkedHashMap, it's noteworthy that both keySet() and values() methods hold true to the same order established by the internal linked list. Regardless of which method you use to traverse the map, the order remains consistent.

Exploring the record: EntrySet

entrySet() gives you a set of Map.Entry objects that depict keys (stops) and their corresponding values (sights). The consistency in this order is maintained by overriding the addEntry() method from HashMap with a twist of linked list magic, ingrained in LinkedHashMap.

These classes share their lineage with LinkedHashMap, but with different ordering mechanisms.

TreeMap: A sorted affair

TreeMap class with its implementation of SortedMap interface ensures an order based on the natural ordering of keys or Comparator provided, giving a feel of arranging books alphabetically.

HashMap: The unpredictable one

On the other hand, HashMap is like a reckless teenager with no guarantee for the order of its elements.

Decoding the Source: Diving into LinkedHashMap

Adventuring into the source code of LinkedHashMap helps in understanding the working mechanisms. The addEntry(...) and linkEntry(...) methods emphasize the engineering marvel that delivers the promise of order.

Meet the siblings: LinkedHashSet and TreeSet

LinkedHashSet and TreeSet are other members of this family that maintain an ordered collection of elements. They are akin to the key set of our LinkedHashMap with a shared traits of orderliness.

Practical applications and caveats

Building LRU caches: A popular task

LinkedHashMap finds its application in implementing LRU caches, where the insertion order plays a critical role in the eviction process.

The Concurrency aspect

Although LinkedHashMap is not thread-safe, it can be made safe using a wrapper by Collections.synchronizedMap() method. You might consider ConcurrentHashMap if concurrent access without order maintenance suffices your needs.

Serialization: Maintaining the order

When a LinkedHashMap is serialized and then deserialized, even on a different JVM instance, the insertion order is preserved. Thus ensuring data consistency across systems.

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