Get enum by its inner field

Question

Hi!

Have enum with inner fields, kind of map.

Now I need to get enum by its inner field.

Wrote this:

package test;

/**
 * Test enum to test enum =)
 */
public enum TestEnum {
    ONE(1), TWO(2), THREE(3);

    private int number;

    TestEnum(int number) {
        this.number = number;
    }      

    public TestEnum findByKey(int i) {
        TestEnum[] testEnums = TestEnum.values();
        for (TestEnum testEnum : testEnums) {
            if (testEnum.number == i) {
                return testEnum;
            }
        }
        return null;
    }
}

But it's not very efficient to look up through all enums each time I need to find appropriate instance.

Is there any other way to do the same?

Answer 1

how about:

TestEnum[] arr = new TestEnum[]{null, ONE,TWO,THREE};

TestEnum one = arr[1];

Answer 2

You should have a HashMap with the numbers as keys and the enum values as values.

This map can typically be in your repository. Then you can easily replace an int variable from the database with your preferred enum value.

If your keys (int values) are stored in a database, then I will say its bad design to carry those keys around in an enum on your business layer. If that's the case, I will recommend not to store the int value in the enum.

Answer 3

You can use a static Map<Integer,TestEnum> with a static initializer that populates it with the TestEnum values keyed by their number fields.

Note that findByKey has been made static, and number has also been made final.

import java.util.*;

public enum TestEnum {
    ONE(1), TWO(2), SIXTY_NINE(69);

    private final int number;    
    TestEnum(int number) {
        this.number = number;
    }

    private static final Map<Integer,TestEnum> map;
    static {
        map = new HashMap<Integer,TestEnum>();
        for (TestEnum v : TestEnum.values()) {
            map.put(v.number, v);
        }
    }
    public static TestEnum findByKey(int i) {
        return map.get(i);
    }

    public static void main(String[] args) {
        System.out.println(TestEnum.findByKey(69)); // prints "SIXTY_NINE"

        System.out.println(
            TestEnum.values() == TestEnum.values()
        ); // prints "false"
    }
}

You can now expect findByKey to be a O(1) operation.

References

• JLS 8.7 Static initializers
• JLS 8.9 Enums

Related questions

• Static initalizer in Java
• How to Initialise a static Map in Java

---

Note on values()

The second println statement in the main method is revealing: values() returns a newly allocated array with every invokation! The original O(N) solution could do a little better by only calling values() once and caching the array, but that solution would still be O(N) on average.

Answer 4

One solution is to add

public final Test[] TESTS = { null, ONE, TWO, THREE };

public static Test getByNumber(int i) {
    return TESTS[i];
}

To the enum.

If the internal data is not an integer, you could have a Map which you populate in a static { ... } initializer. This map could later be used in the getByNumber method above.

Answer 5

Although someone has suggested using Map<Integer, TestEnum> think twice about it.

Your original solution, especially for small enums, may be magnitudes faster than using HashMap.

HashMap will probably be not faster until your enum contains at least 30 to 40 elements.

This is one case of "If it ain't broken, don't fix it".