Get a map from enum in Java

In this post, I am going to explain how to get a Map from an enum.

private static enum Month {
    JANUARY(1), 
    FEBRUARY(2), 
    MARCH(3), 
    APRIL(4), 
    MAY(5), 
    JUNE(6), 
    JULY(7), 
    AUGUST(8), 
    SEPTEMBER(9), 
    OCTOBER(10),
    NOVEMBER(11), 
    DECEMBER(12);

    private final int index;

    Month(int index) {
        this.index = index;
    }

    public int getIndex() {
        return index;
    }

}

 

As you see above snippet, Month enum defines all the months and their indexes.

Below snippet get a Map from the Month enum, where Month index is the key and Month as a value.

Map<Integer, Month> monthsByIndex = unmodifiableMap(Arrays.stream(Month.values()).collect(Collectors.toMap(Month::getIndex, identity())));

 

Find the below working application.

 

MapFromEnum.java

package com.sample.app.collections;

import static java.util.Collections.unmodifiableMap;
import static java.util.function.Function.identity;

import java.util.Arrays;
import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collectors;

public class MapFromEnum {
    private static enum Month {
        JANUARY(1), FEBRUARY(2), MARCH(3), APRIL(4), MAY(5), JUNE(6), JULY(7), AUGUST(8), SEPTEMBER(9), OCTOBER(10),
        NOVEMBER(11), DECEMBER(12);

        private final int index;

        Month(int index) {
            this.index = index;
        }

        public int getIndex() {
            return index;
        }

    }

    public static void main(String[] args) {
        Map<Integer, Month> monthsByIndex = unmodifiableMap(
                Arrays.stream(Month.values()).collect(Collectors.toMap(Month::getIndex, identity())));

        for (Entry<Integer, Month> entry : monthsByIndex.entrySet()) {
            System.out.println(entry.getKey() + "\t" + entry.getValue());
        }
    }
}

 

Output

1   JANUARY
2   FEBRUARY
3   MARCH
4   APRIL
5   MAY
6   JUNE
7   JULY
8   AUGUST
9   SEPTEMBER
10  OCTOBER
11  NOVEMBER
12  DECEMBER

 

 

 

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