Java EnumSet

The EnumSet class of the Java collections framework provides a set implementation of elements of a single enum.

Before you learn about EnumSet, make sure to know about Java Enums.

It implements the Set interface.

Creating EnumSet

In order to create an enum set, we must import the java.util.EnumSet package first.

Unlike other set implementations, the enum set does not have public constructors. We must use the predefined methods to create an enum set.

1. Using allOf(Size)

The allof() method creates an enum set that contains all the values of the specified enum type Size. For example,

import java.util.EnumSet;

class Main {
    // an enum named Size
    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }
    
    public static void main(String[] args) {

        // Creating an EnumSet using allOf()
        EnumSet<Size> sizes = EnumSet.allOf(Size.class);

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

}

Output

EnumSet: [SMALL, MEDIUM, LARGE, EXTRALARGE]

Notice the statement,

EnumSet<Size> sizes = EnumSet.allOf(Size.class);

Here, Size.class denotes the Size enum that we have created.

2. Using noneOf(Size)

The noneOf() method creates an empty enum set. For example,

import java.util.EnumSet;

class Main {

     // an enum type Size
    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }

    public static void main(String[] args) {

        // Creating an EnumSet using noneOf()
        EnumSet<Size> sizes = EnumSet.noneOf(Size.class);

        System.out.println("Empty EnumSet: " + sizes);
    }
}

Output

Empty EnumSet : []

Here, we have created an empty enum named sizes.

Note: We can only insert elements of enum type Size in the above program. It's because we created our empty enum set using Size enum.

3. Using range(e1, e2) Method

The range() method creates an enum set containing all the values of an enum between e1 and e2 including both values. For example,

import java.util.EnumSet;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }

    public static void main(String[] args) {

        // Creating an EnumSet using range()
        EnumSet<Size> sizes = EnumSet.range(Size.MEDIUM, Size.EXTRALARGE);

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

Output

EnumSet: [MEDIUM, LARGE, EXTRALARGE]

4. Using of() Method

The of() method creates an enum set containing the specified elements. For example,

import java.util.EnumSet;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }

    public static void main(String[] args) {

        // Using of() with a single parameter
        EnumSet<Size> sizes1 = EnumSet.of(Size.MEDIUM);
        System.out.println("EnumSet1: " + sizes1);

        EnumSet<Size> sizes2 = EnumSet.of(Size.SMALL, Size.LARGE);
        System.out.println("EnumSet2: " + sizes2);
    }
}

Output

EnumSet1: [MEDIUM]
EnumSet2: [SMALL, LARGE]

Methods of EnumSet

The EnumSet class provides methods that allow us to perform various elements on the enum set.

Insert Elements to EnumSet

• add() - inserts specified enum values to the enum set
• addAll() inserts all the elements of the specified collection to the set

For example,

import java.util.EnumSet;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }

    public static void main(String[] args) {

        // Creating an EnumSet using allOf()
        EnumSet<Size> sizes1 = EnumSet.allOf(Size.class);

        // Creating an EnumSet using noneOf()
        EnumSet<Size> sizes2 = EnumSet.noneOf(Size.class);

        // Using add method
        sizes2.add(Size.MEDIUM);
        System.out.println("EnumSet Using add(): " + sizes2);

        // Using addAll() method
        sizes2.addAll(sizes1);
        System.out.println("EnumSet Using addAll(): " + sizes2);
    }
}

Output

EnumSet using add(): [MEDIUM]
EnumSet using addAll(): [SMALL, MEDIUM, LARGE, EXTRALARGE]

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

It's also possible to insert elements from other collections such as ArrayList, LinkedList, etc. to an enum set using addAll(). However, all collections should be of the same enum type.

Access EnumSet Elements

To access elements of an enum set, we can use the iterator() method. In order to use this method, we must import the java.util.Iterator package. For example,

import java.util.EnumSet;
import java.util.Iterator;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }

    public static void main(String[] args) {

        // Creating an EnumSet using allOf()
        EnumSet<Size> sizes = EnumSet.allOf(Size.class);

        Iterator<Size> iterate = sizes.iterator();

        System.out.print("EnumSet: ");
        while(iterate.hasNext()) {
            System.out.print(iterate.next());
            System.out.print(", ");
        }
    }
}

Output

EnumSet: SMALL, MEDIUM, LARGE, EXTRALARGE,

Note:

• hasNext() returns true if there is a next element in the enum set
• next() returns the next element in the enum set

Remove EnumSet Elements

• remove() - removes the specified element from the enum set
• removeAll() - removes all the elements from the enum set

For example,

import java.util.EnumSet;

class Main {

    enum Size {
        SMALL, MEDIUM, LARGE, EXTRALARGE
    }

    public static void main(String[] args) {

        // Creating EnumSet using allOf()
        EnumSet<Size> sizes = EnumSet.allOf(Size.class);
        System.out.println("EnumSet: " + sizes);

        // Using remove()
        boolean value1 = sizes.remove(Size.MEDIUM);
        System.out.println("Is MEDIUM removed? " + value1);

        // Using removeAll()
        boolean value2 = sizes.removeAll(sizes);
        System.out.println("Are all elements removed? " + value2);
    }
}

Output

EnumSet: [SMALL, MEDIUM, LARGE, EXTRALARGE]
Is MEDIUM removed? true
Are all elements removed? true

Other Methods

Clonable and Serializable Interfaces

The EnumSet class also implements Cloneable and Serializable interfaces.

Cloneable Interface

It allows the EnumSet 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.

Why EnumSet?

The EnumSet provides an efficient way to store enum values than other set implementations (like HashSet, TreeSet).

An enum set only stores enum values of a specific enum. Hence, the JVM already knows all the possible values of the set.

This is the reason why enum sets are internally implemented as a sequence of bits. Bits specifies whether elements are present in the enum set or not.

The bit of a corresponding element is turned on if that element is present in the set.