What is an EnumMap in Java?

This article will explain what an enumMap is, and how to use it in Java.

What is an EnumMap?

An EnumMap is a special form of a map. A map in Java is an interface that takes a key (K) and a value (V).

In the case of an EnumMap, the key is an enum.

An EnumMap:

• extends the AbstractMap.

• implements Map interface.

How to Create an enumMap

To create an enumMap, we have to import the java.util.EnumMap

public class JavaEnumMap {

    enum Colour {
        RED,
        BLUE,
        BLACK,
        GREEN
    }
}

How to Add an Element Into an Enummap

You can use the put() method to insert elements into an enumMap.

    public static void main(String[] args) {

        EnumMap<Colour, Integer> colours = new EnumMap<Colour, Integer>(Colour.class);

        colours.put(Colour.RED, 1);
        colours.put(Colour.BLUE, 2);
        colours.put(Colour.BLACK, 3);
        colours.put(Colour.GREEN, 4);
}

How to Access an Element on an Enummap

To access the elements of an enumMap, you can use:

keySet()

It prints all the keys on the map

System.out.println("Printing all keys in the map " + colours.keySet());

The result is:

Printing all keys in the map [RED, BLUE, BLACK, GREEN]

entrySet()

It prints the keys and the values in the map

System.out.println("Printing the keys and the values in the map " + colours.entrySet());

The result is:

Printing the keys and the values in the map [RED=1, BLUE=2, BLACK=3, GREEN=4]

values()

It prints the values in the map

System.out.println("Printing the values in the map " + colours.values());

The result is:

Printing the values in the map [1, 2, 3, 4]

How to Retrieve Elements From an Enummap

You can use the get() method to retrieve elements from an enumMap.

System.out.println("Retrieve the value from the enumMap " + colours.get(Colour.RED));

The result is:

Retrieve the value from the enumMap: 1

How to Remove Elements From an Enummap

To remove elements from an enumMap you can use:

remove(key)

You use the key of the enumMap to remove an element from an enumMap

System.out.println("Removing the key: " + colours.remove(Colour.GREEN));

The result is:

Removing the key: 4
The enumMap now has: {RED=1, BLUE=2}

remove(key, value)

If it exists, it removes the element from the enumMap and returns a boolean value.

System.out.println("Removing the key and the value from the enumMap: " + colours.remove(Colour.BLACK, 3));
Removing the key and the value from the enumMap: true
The enumMap now has: {RED=1, BLUE=2}

The result is:

The enumMap now has: {RED=1, BLUE=2}

How to replace an enumMap

replace(key, value)

It replaces the old value with a new value.

colours.replace(Colour.RED, 5);

System.out.println("The new enum now has: " + colours);

Result is:

The new enum now has: {RED=5, BLUE=2}

replace (key, oldValue, newValue)

This method replaces the old value with a new one only if the old value is linked with a specific key.

colours.replace(Colour.BLUE, 2, 4);

System.out.println("The new enum now has: " + colours);

The result is:

The new enum now has: {RED=5, BLUE=4}

replaceAll()

It replaces the value of each element with the result of the function.

      colours.replaceAll((key, oldValue) -> oldValue + 7);

        System.out.println("EnumMap using replaceAll(): " + colours);

The result is:

EnumMap using replaceAll(): {RED=12, BLUE=11}

A few notes about enumMaps:

• EnumMaps maintains the natural order of their keys.

• We cannot have null keys.

• Internally, enumMaps are represented as arrays.

CONCLUSION

In this article, you've learned about enumMaps and its methods.

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ADDITIONAL RESOURCES

• JavaDoc: EnumMap

• Baeldung: A Guide to EnamMap

• Ritesh Panigrahi: Complete Guide to Java Enum