Java EnumMap

The EnumMap class of the Java collections framework provides a map implementation for elements of an enum.

In EnumMap, enum elements are used as keys. It implements the Map interface.

Before we learn about EnumMap, make sure to know about the Java Enums.

Creating an EnumMap

In order to create an enum map, we must import the java.util.EnumMap package first. Once we import the package, here is how we can create enum maps in Java.

enum Size {
    SMALL, MEDIUM, LARGE, EXTRALARGE
}

EnumMap<Size, Integer> sizes = new EnumMap<>(Size.class);

In the above example, we have created an enum map named sizes.

Here,

• Size - keys of the enum that map to values
• Integer - values of the enum map associated with the corresponding keys

Methods of EnumMap

The EnumMap class provides methods that allow us to perform various elements on the enum maps.

Insert Elements to EnumMap

• put() - inserts the specified key/value mapping (entry) to the enum map
• putAll() - inserts all the entries of a specified map to this map

For example,

import java.util.EnumMap;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }
    public static void main(String[] args) {

        // Creating an EnumMap of the Size enum
        EnumMap<Size, Integer> sizes1 = new EnumMap<>(Size.class);

        // Using the put() Method
        sizes1.put(Size.SMALL, 28);
        sizes1.put(Size.MEDIUM, 32);
        System.out.println("EnumMap1: " + sizes1);

        EnumMap<Size, Integer> sizes2 = new EnumMap<>(Size.class);

        // Using the putAll() Method
        sizes2.putAll(sizes1);
        sizes2.put(Size.LARGE, 36);
        System.out.println("EnumMap2: " + sizes2);
    }
}

Output

EnumMap1: {SMALL=28, MEDIUM=32}
EnumMap2: {SMALL=28, MEDIUM=32, LARGE=36}

In the above example, we have used the putAll() method to insert all the elements of an enum map sizes1 to an enum map of sizes2.

It is also possible to insert elements from other maps such as HashMap, TreeMap, etc. to an enum map using putAll(). However, all maps should be of the same enum type.

Access EnumMap Elements

1. Using entrySet(), keySet() and values()

• entrySet() - returns a set of all the keys/values mapping (entry) of an enum map
• keySet() - returns a set of all the keys of an enum map
• values() - returns a set of all the values of an enum map

For example,

import java.util.EnumMap;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }
    public static void main(String[] args) {

        // Creating an EnumMap of the Size enum
        EnumMap<Size, Integer> sizes = new EnumMap<>(Size.class);
        sizes.put(Size.SMALL, 28);
        sizes.put(Size.MEDIUM, 32);
        sizes.put(Size.LARGE, 36);
        sizes.put(Size.EXTRALARGE, 40);
        System.out.println("EnumMap: " + sizes);

        // Using the entrySet() Method
        System.out.println("Key/Value mappings: " + sizes.entrySet());

        // Using the keySet() Method
        System.out.println("Keys: " + sizes.keySet());

        // Using the values() Method
        System.out.println("Values: " + sizes.values());
    }
}

Output

EnumMap: {SMALL=28, MEDIUM=32, LARGE=36, EXTRALARGE=40}
Key/Value mappings: [SMALL=28, MEDIUM=32, LARGE=36, EXTRALARGE=40]
Keys: [SMALL, MEDIUM, LARGE, EXTRALARGE]
Values: [28, 32, 36, 40]

2. Using the get() Method

The get() method returns the value associated with the specified key. It returns null if the specified key is not found.

For example,

import java.util.EnumMap;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }
    public static void main(String[] args) {

        // Creating an EnumMap of the Size enum
        EnumMap<Size, Integer> sizes = new EnumMap<>(Size.class);
        sizes.put(Size.SMALL, 28);
        sizes.put(Size.MEDIUM, 32);
        sizes.put(Size.LARGE, 36);
        sizes.put(Size.EXTRALARGE, 40);
        System.out.println("EnumMap: " + sizes);

        // Using the get() Method
        int value = sizes.get(Size.MEDIUM);
        System.out.println("Value of MEDIUM: " + value);
    }
}

Output

EnumMap: {SMALL=28, MEDIUM=32, LARGE=36, EXTRALARGE=40}
Value of MEDIUM: 32

Remove EnumMap Elements

• remove(key) - returns and removes the entry associated with the specified key from the map
• remove(key, value) - removes the entry from the map only if the specified key mapped to the specified value and return a boolean value

For example,

import java.util.EnumMap;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }
    public static void main(String[] args) {

        // Creating an EnumMap of the Size enum
        EnumMap<Size, Integer> sizes = new EnumMap<>(Size.class);
        sizes.put(Size.SMALL, 28);
        sizes.put(Size.MEDIUM, 32);
        sizes.put(Size.LARGE, 36);
        sizes.put(Size.EXTRALARGE, 40);
        System.out.println("EnumMap: " + sizes);

        // Using the remove() Method
        int value = sizes.remove(Size.MEDIUM);
        System.out.println("Removed Value: " + value);

        boolean result = sizes.remove(Size.SMALL, 28);
        System.out.println("Is the entry {SMALL=28} removed? " + result);

        System.out.println("Updated EnumMap: " + sizes);
    }
}

Output

EnumMap: {SMALL=28, MEDIUM=32, LARGE=36, EXTRALARGE=40}
Removed Value: 32
Is the entry {SMALL=28} removed? True
Updated EnumMap: {LARGE=36, EXTRALARGE=40}

Replace EnumMap Elements

• replace(key, value) - replaces the value associated with the specified key by the new value
• replace(key, old, new) - replaces the old value with the new value only if the old value is already associated with the specified key
• replaceAll(function) - replaces each value of the map with the result of the specified function

import java.util.EnumMap;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }
    public static void main(String[] args) {

        // Creating an EnumMap of the Size enum
        EnumMap<Size, Integer> sizes = new EnumMap<>(Size.class);
        sizes.put(Size.SMALL, 28);
        sizes.put(Size.MEDIUM, 32);
        sizes.put(Size.LARGE, 36);
        sizes.put(Size.EXTRALARGE, 40);
        System.out.println("EnumMap: " + sizes);

        // Using the replace() Method
        sizes.replace(Size.MEDIUM, 30);
        sizes.replace(Size.LARGE, 36, 34);
        System.out.println("EnumMap using replace(): " + sizes);

        // Using the replaceAll() Method
        sizes.replaceAll((key, oldValue) -> oldValue + 3);
        System.out.println("EnumMap using replaceAll(): " + sizes);
    }
}

Output

EnumMap: {SMALL=28, MEDIUM=32, LARGE=36, EXTRALARGE=40}
EnumMap using replace(): {SMALL=28, MEDIUM=30, LARGE=34, EXTRALARGE=40}
EnumMap using replaceAll(): {SMALL=31, MEDIUM=33, LARGE=37, EXTRALARGE=43}

In the above program, notice the statement

sizes.replaceAll((key, oldValue) -> oldValue + 3);

Here, the method accesses all the entries of the map. It then replaces all the values with the new values provided by the lambda expressions.

Other Methods

EnumSet Vs. EnumMap

Both the EnumSet and EnumMap class provides data structures to store enum values. However, there exist some major differences between them.

• Enum set is represented internally as a sequence of bits, whereas the enum map is represented internally as arrays.
• Enum set is created using its predefined methods like allOf(), noneOf(), of(), etc. However, an enum map is created using its constructor.

Clonable and Serializable Interfaces

The EnumMap class also implements Cloneable and Serializable interfaces.

Cloneable Interface

It allows the EnumMap class to make a copy of instances of the class.

Serializable Interface

Whenever Java objects need to be transmitted over a network, objects need to be converted into bits or bytes. This is because Java objects cannot be transmitted over the network.

The Serializable interface allows classes to be serialized. This means objects of the classes implementing Serializable can be converted into bits or bytes.