Java EnumSet Class

Quiz

Introduction

The Java EnumSet class is a specialized Set implementation for use with enum types. The EnumSet is a part of java.util package that provides a highly optimized set of enumeration types.

EnumSet is not synchronized, and to make it thread-safe, synchronization is needed externally. It also implements the Serializable and the Cloneable interfaces.

Characterstics

The following are the important points about the EnumSet class in Java −

All of the elements in an enum set must come from a "single enum type" that is specified, explicitly or implicitly, when the set is created.
Enum sets are represented internally as bit vectors.
The EnumSet class does not allow Null elements, and it will throw a NullPointerException error if we try to insert Null elements.
The EnumSet class uses a fail-safe iterator that works on a copy, so modifying a collection while iterating over it won't throw a ConcurrentModificationException error.

Class Declaration

The following is the declaration for java.util.EnumSet class −

public abstract class EnumSet<E extends Enum<E>>
   extends AbstractSet<E>
   implements Cloneable, Serializable

Parameters

E − This is the type of elements maintained by this set.

Why use an EnumSet?

The EnumSet provides a better way to store enum values as compared to other implementations like HashSet or TreeSet. An EnumSet only stores enum values of a specific enum. Hence, the JVM(Java Virtual Machine) already knows all the possible values of that set.

This is why enum sets are internally implemented as a sequence of bits. Because bits specify whether elements are present in the EnumSet or not. The bit corresponding to the element is turned on if that element is present in the set.

Implementing an EnumSet

The EnumSet is an abstract class, so we can not directly create an instance of it. Instead, it provides static factory methods to create instances of the EnumSet.

The JDK provides 2 different implementations for the EnumSet as follows −

RegularEnumSet − It uses a single long to represent the bit vector. Each bit in the long value represents an enum value. As we know, that long data type can store 64 bits, so the RegularEnumSet can store upto 64 enum values.
JumboEnumSet − It uses an array of long elements as a bit vector. The main advantage of the JumboEnumSet is that it can store more than 64 enum values.

If the number of elements (enumValues.length) is less than or equal to 64, the RegularEnumSet is used otherwise, the JumboEnumSet is used for larger enum types.

if (enumValues.length <= 64)
    return new RegularEnumSet<>(elementType, enumValues);
else
    return new JumboEnumSet<>(elementType, enumValues);

It takes into account only the size of the enum class, not the number of elements that will be stored in the collection.

We will be using the predefined methods to create an EnumSet. The Java EnumSet of(E) method is used to populate the EnumSet instance. We've created an enum Numbers. Then an EnumSet instance is created using an enum value, and the resulting EnumSet is printed.

Example of Creating an EnumSet

Below is an example of creating an EnumSet using the of() method in Java −

package com.tutorialspoint;

import java.util.EnumSet;

public class EnumSetDemo {

   // create an enum
   public enum Numbers {
      ONE, TWO, THREE, FOUR, FIVE
   };

   public static void main(String[] args) {

      // create a set that contains an enum
      EnumSet<Numbers> set = EnumSet.of(Numbers.ONE);

      // print set
      System.out.println("Set:" + set);
   }
}

Let us compile and run the above program. This will produce the following result −

Set:[ONE]

Class Methods

The following are the class methods supported by the EnumSet in Java −

Sr.No.	Method & Description
1	static <E extends Enum<E>> EnumSet<E> allOf(Class<E> elementType) This method creates an enum set containing all of the elements in the specified element type.
2	EnumSet<E> clone() This method returns a copy of this set.
3	static <E extends Enum<E>> EnumSet<E> complementOf(EnumSet<E> s) This method creates an enum set with the same element type as the specified enum set, initially containing all the elements of this type that are not contained in the specified set.
4	static <E extends Enum<E>> EnumSet<E> copyOf(Collection<E> c) This method creates an enum set initialized from the specified collection.
5	static <E extends Enum<E>> EnumSet<E> copyOf(EnumSet<E> s) This method creates an enum set with the same element type as the specified enum set, initially containing the same elements (if any).
6	static <E extends Enum<E>> EnumSet<E> noneOf(Class<E> elementType) This method creates an empty enum set with the specified element type.
7	static <E extends Enum<E>> EnumSet<E> of(E e) This method creates an enum set initially containing the specified element.
8	static <E extends Enum<E>> EnumSet<E> range(E from, E to) This method creates an enum set initially containing all of the elements in the range defined by the two specified endpoints.

Methods inherited

The EnumSet class inherits methods from the following classes −

java.util.AbstractSet
java.util.AbstractCollection
java.util.Object
java.util.Set

Creating EnumSet Instances using allOf(), noneOf(), and range() Methods

The Java EnumSet provides other methods like allOf(), noneOf(), and range() to create instances for the EnumSet. The allOf(E) method includes all the enum values present in the EnumSet, and the noneOf(E) method creates an empty EnumSet for the given set, and the range(E1, E2) method includes values between the given range(starting from E1 and ending at E2).

Example

Below is an example creating an EnumSet using allOf(), noneOf(), and range() methods in Java −

package com.tutorialspoint;

import java.util.EnumSet;

public class EnumSetDemo {

   // create an enum
   public enum Numbers {
      ONE, TWO, THREE, FOUR, FIVE
   }

   public static void main(String[] args) {

      // using the allOf() method to print all enum values
      EnumSet<Numbers> all_Numbers = EnumSet.allOf(Numbers.class);
      System.out.println("All enum values: " + all_Numbers);

      // using the noneOf() method to creates an empty set
      EnumSet<Numbers> empty_Set = EnumSet.noneOf(Numbers.class);
      System.out.println("Empty set: " + empty_Set);
      
       // using the range() method to print between a given range 
      EnumSet<Numbers> range_Set = EnumSet.range(Numbers.TWO, Numbers.FOUR);
      System.out.println("Range set: " + range_Set);     
   }
}

Output

Let us compile and run the above program. This will produce the following result −

All enum values: [ONE, TWO, THREE, FOUR, FIVE]
Empty set: []
Range set: [TWO, THREE, FOUR]

Adding values to an EnumSet

The EnumSet add() method is used to add entries to the EnumSet, and the addAll() method is used to add the existing values of the EnumSet to a new EnumSet. We've created an EnumSet of "Numbers". Then few entries are added using the add() method, and then the EnumSet is printed. Then, using the addAll() method, the values were added to a new EnumSet, and the set was printed.

Example

Below is an example of adding values to an EnumSet using the add() and addAll() methods in Java −

package com.tutorialspoint;

import java.util.EnumSet;

public class EnumSetDemo {

    // create an enum
    public enum Numbers {
        ONE, TWO, THREE, FOUR, FIVE
    }

    public static void main(String[] args) {

        // creating EnumSet with allOf() and noneOf() methods
        EnumSet<Numbers> set = EnumSet.allOf(Numbers.class);
        EnumSet<Numbers> newSet = EnumSet.noneOf(Numbers.class);

        // using the add() method
        newSet.add(Numbers.ONE);
        System.out.println("After adding the value: " + newSet);

        // using the addAll() method
        newSet.addAll(set);
        System.out.println("After adding all the values: " + newSet);
    }
}

Output

Let us compile and run the above program. This will produce the following result −

After adding the value: [ONE]
After adding all the values: [ONE, TWO, THREE, FOUR, FIVE]

Iterating values of an EnumSet

The EnumSet provides the iterator() method for accessing the elements from an EnumSet. We've created an EnumSet of "Numbers". Then the iterator() method is used to print the set elements.

Example

Below is an example of iterating over the values of an EnumSet using the iterator() method in Java −

package com.tutorialspoint;

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

public class EnumSetDemo {

    // create an enum
    public enum Numbers {
        ONE, TWO, THREE, FOUR, FIVE
    }

    public static void main(String[] args) {

        // creating an EnumSet using the of() method
        EnumSet<Numbers> numbers = EnumSet.allOf(Numbers.class);

        // using iterator() method to print the EnumSet
        Iterator<Numbers> iterate = numbers.iterator();
        while (iterate.hasNext()) {
            System.out.println(iterate.next());
        }
    }
}

Output

Let us compile and run the above program. This will produce the following result.

ONE
TWO
THREE
FOUR
FIVE

Removing values from an EnumSet

The EnumSet remove() method is used to remove entries from the EnumSet, and the removeAll() method is used to remove all the existing values from the EnumSet. We've created an EnumSet of "Numbers". Then few entries are removed using the remove() method, and then the EnumSet is printed. Then, using the removeAll() method, the set is cleared and checked if the EnumSet is empty.

Example

Below is an example of removing values from an EnumSet using the remove() and removeAll() methods in Java −

package com.tutorialspoint;

import java.util.EnumSet;

public class EnumSetDemo {

    // create an enum
    public enum Numbers {
        ONE, TWO, THREE, FOUR, FIVE
    }

    public static void main(String[] args) {

        // Creating an EnumSet with allOf() method
        EnumSet<Numbers> numbers = EnumSet.allOf(Numbers.class);
        System.out.println("EnumSet: " + numbers);

        // using the remove() method
        numbers.remove(Numbers.TWO);
        System.out.println("EnumSet after deletion:  " + numbers);

        //  using the removeAll() method
        numbers.removeAll(numbers);
        System.out.println("EnumSet is empty: " + numbers.isEmpty());
    }
}

Output

Let us compile and run the above program. This will produce the following result −

EnumSet: [ONE, TWO, THREE, FOUR, FIVE]
EnumSet after deletion:  [ONE, THREE, FOUR, FIVE]
EnumSet is empty: true

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