Thursday, April 14, 2011

Simple Reverse Lookup Enum in Java

Documentation inline with the code.

import java.util.EnumSet;
import java.util.HashMap;
import java.util.Map;

 * @author Intesar Mohammed
 *  Reverse Enum Lookup example using Map and EnumSet
public enum Status {


    *   code or your required variable can be of any type
    private int code;

    private Status(int code) {
        this.code = code;
    private static final Map<Integer, Status> lookup = 
                  new HashMap<Integer, Status>();
     * Stores all Enum's into lookup
    static {
        for (Status s : EnumSet.allOf(Status.class)) {
            lookup.put(s.code, s);

     * toString() is the only api to print Enum value
     * All Enum classes can consistently override toString() and provide a simple api
     * across different Enums
     * @return
    public String toString() {
        return String.valueOf(code);

     * Sample lookup code
     * Status status = Status.get(0);
     * Status status = Status.get(5);
     * Status status = Status.get(10); // this will return null
     * Also you can rename get() to reverseLookup() or lookup()
     * for readability convenience
     * @param code
     * @return
    public static Status get(int code) {
        return lookup.get(code);



Anonymous said...

What's the advantage of using EnumSet rather than values()?

Eric Jablow said...

This won't compile unless you use generics: Map<Integer, Status> and HashMap<Integer, Status>.

It seems like unnecessary overhead, but I prefer to wrap such maps with Collections.unmodifiableMap(). It makes the intent clear.

joonas said...

Even better would be to use EnumMap instead of HashMap.

tsachev said...

I usually do the static initialisation in a separate inner static class. I know it is not very pretty, but that way you should not bother about order of your declarations.

mtrovo said...

This is very useful when you need to use Enums with a database field with checked constraint.

Do you know the class EnumMap?
It's a map optimized for Enums and it's very suitable for what you're trying to do.


Intesar said...

EnumMap is not a good choice here as its a specialized Map implementation for use with enum type keys

also checkout jdk1.6 documentation

JAvin @ HashTable vs HashMap in Java said...

would have better if you could explain some usecases around it anyway something new to learn.

How Garbage collection works in Java

scardis said...

I do this type "lookup by code" logic in my enum(s) quite often, but rather than use a static block outside of the constructor, I add it to the static "lookup" Map in the constructor itself.

Sandeep said...

Thanks for this great article
Extreme Java

JP @ java debugging tutorial said...

Indeed a useful one. Enum in Java are great feature and most versatile and very useful under certain scenario. In my opinion following are some of benefits of enum in java :
Best feature of Enum is you can use Enum in Java inside Switch statement like int or char primitive data type.we will also see example of using java enum in switch statement in this java enum tutorial.
By the way I have also blogged my experience as 10 examples of enum in java , let me know how do you find it.