Java 21 LinkedHashSet.addFirst and addLast relocate existing elements

Last updated: 5 Oct 20264 min read

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LinkedHashSet.addFirst and addLast can move an existing member to an encounter-order endpoint without creating a duplicate.

Treat membership and order separately

A Set still contains one occurrence of each equal element. The Java 21 endpoint methods can reposition that occurrence: addFirst moves a member to the front, and addLast moves it to the back. Ordinary add of an existing member does not express that relocation. This matters when a deduplicated processing queue also has a display or retry order.

The fixture moves one dispatch ID forward and another backward, then removes the last. The final size checks membership separately from order. A duplicate value is not a second node.

Use a mutable implementation deliberately

SequencedSet describes encounter order but does not promise that every implementation supports repositioning. LinkedHashSet does. If the application needs key/value pairs as well, LinkedHashMap endpoint placement provides the analogous map operation.

Element equality and hash codes must remain stable while stored in a hash-based set. Mutating fields used by hashCode can make a member unreachable through ordinary lookup even though iteration still shows it.

Working program

import java.util.LinkedHashSet;
import java.util.List;

public class ReorderDispatchIds {
    public static void main(String[] args) {
        LinkedHashSet<String> dispatchIds = new LinkedHashSet<>(
            List.of("load-15", "seal-47", "ship-82"));
        dispatchIds.addFirst("ship-82");
        dispatchIds.addLast("load-15");
        System.out.println(dispatchIds);
        System.out.println(dispatchIds.removeLast());
        System.out.println(dispatchIds.size());
    }
}

Output

[ship-82, seal-47, load-15]
load-15
2

Cost and ownership

Repositioning does not allocate a second entry for an existing member. Hash-based membership operations are expected constant-time under ordinary hash distribution; iterating the order remains O(n).

Common Mistakes

Do not expect two copies of the same set member after relocation.
Do not assume every SequencedSet implementation supports endpoint mutation.
Do not mutate fields used in an element's equality or hash code while it is in a hash set.