Java EnumMap: declaration order and nullable values

Last updated: 30 Sept 20264 min read

tutorial

EnumMap keys are enum constants iterated in their declaration order, while null values are allowed and null keys are not.

Ordering does not follow insertion

The fixture inserts REVIEW before RECEIVED. Iteration still reports RECEIVED first because the enum declaration fixes key order. A REVIEW entry with a null value remains present. containsKey distinguishes that state from an absent mapping; get alone cannot.

The domain still needs a meaning for null. An enum state map can use an explicit marker or Optional-like result instead of an ambiguous null value. Enum collections covers when EnumMap and EnumSet fit.

The key type is part of the instance

Construct the map with the enum class or from a nonempty compatible source. A null key is rejected. This local map is not synchronized and does not provide cross-thread state transitions.

Working program

import java.util.EnumMap;
import java.util.Map;
public class ReceiptStageNotes {
    enum Stage { RECEIVED, REVIEW, SENT }
    public static void main(String[] args) {
        Map<Stage, String> notes = new EnumMap<>(Stage.class);
        notes.put(Stage.REVIEW, null);
        notes.put(Stage.RECEIVED, "logged");
        for (Stage stage : notes.keySet()) System.out.println(stage);
        System.out.println("review present=" + notes.containsKey(Stage.REVIEW));
        System.out.println("review note=" + notes.get(Stage.REVIEW));
    }
}

Output

RECEIVED
REVIEW
review present=true
review note=null

Costs and boundaries

EnumMap is specialized for one enum key type; its order is tied to that type's declaration. The example does not establish a persisted state machine or concurrency policy.

Common Mistakes

Do not expect insertion order.
Do not interpret get == null as proof of absence.
Do not insert a null enum key.