EnumSet in Java

EnumSet in Java

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In Java, you use the enumeration type to represent a list of predefined constants. But, what if you want to implement a Set interface with the enumeration type, you have got EnumSet in Java to do so.

Enum constants are unique and have predefined length, as you can not define a new enum constant at runtime, therefore it allows Java API designers to highly optimize EnumSet. It is henceforth clearly stated that an EnumSet is a specialized Set collection to work with enum classes.

If you are not familiar to using enums in Java, you can check my post on Using Java Enums.

In this post, you will learn about using EnumSet in Java, its various methods and their implementation.

Important features of EnumSet

  • EnumSet can contain only enum values and all the values should belong to the same enum.
  • It does not allow adding null objects and throws NullPointerException if we try to do so.
  • It extends AbstractSet class and implements Set Interface in Java.
  • The elements in EnumSet are stored following the order in which they are declared in the enum.
  • It uses a fail-safe iterator that works on a copy, so it won’t throw a ConcurrentModificationException if the collection is modified when iterating over it.
  • It is not thread-safe, so we need to synchronize it externally if required.

Declaring an EnumSet

The declaration for java.util.EnumSet class is this.

public abstract class EnumSet<E extends Enum<E>>  

Here, E specifies the elements. E must extend Enum, which enforces the requirement that the elements must be of the specified enum type.

Creating an 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. The various methods of creating an EnumSet are as follows:

EnumSet using allOf(Size)

The allof() method creates an enum set that contains all the values of the specified enum type Size.
Here is the code for allof() method of creating EnumSet.

EnumSetExample.java

import java.util.EnumSet;

public class EnumSetExample {

    enum Days {
        MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
    }

    public static void main(String[] args) {

        EnumSet days = EnumSet.allOf(Days.class);

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

The preceding code defines an enum type, Days. The names of the enum type’s fields are in uppercase letters as they are constants. The allOf() method is used here to create an enum set that will be used to contain all of the elements in the specified elementType.

The output on running the code is this.

allOfMethod

EnumSet using noneOf(Size)

The noneOf() method creates an empty enum set.

The code for noneOf() method is this.

EnumSetExample.java

import java.util.EnumSet;

public class EnumSetExample {

    enum Days {
        MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
    }

    public static void main(String[] args) {

        EnumSet emptyDays = EnumSet.noneOf(Days.class);

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

Here, the noneOf() method creates a null set of the type elementType.

This is the output of the preceding code.

noneOfMethod

EnumSet 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.

EnumSetExample.java

import java.util.EnumSet;

public class EnumSetExample {

    enum Days {
        MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
    }

    public static void main(String[] args) {
        
       EnumSet rangeDays = EnumSet.range(Days.TUESDAY, Days.FRIDAY);

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

Thus, here Days from TUESDAY to FRIDAY will be added into the enum set.

The output on running the code in IntelliJ is this.

rangeMethod

EnumSet using of() Method

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

The code for of() method is this.

EnumSetExample.java

import java.util.EnumSet;

public class EnumSetExample {

    enum Days {
        MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
    }

    public static void main(String[] args) {
    EnumSet days1 = EnumSet.of(Days.SUNDAY);
        System.out.println("EnumSet1: " + days1);

        EnumSet days2 = EnumSet.of(Days.SATURDAY, Days.SUNDAY);
        System.out.println("EnumSet2: " + days2);
  }
}

Thus, here only the specied days SUNDAY in days1 and SATURDAY and SUNDAY in days2 will be added in the enum set.

Here is the Output.

ofMethod

EnumSet Methods

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

Inserting elements to EnumSet

There are two ways to insert elements in anEnumSet
First is the add() method which inserts specified enum values to the enum set.

Secondly, the addAll() method inserts all the elements of the specified collection to the set.

Here is the code for both add() and addAll() methods.

EnumSetExample.java

import java.util.EnumSet;

public class EnumSetExample {

    enum Days {
        MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
    }

    public static void main(String[] args) {

   EnumSet days1 = EnumSet.of(Days.SUNDAY);
        System.out.println("EnumSet1: " + days1);

        EnumSet days2 = EnumSet.of(Days.SATURDAY, Days.SUNDAY);
        System.out.println("EnumSet2: " + days2);

        // Using add method
        days1.add(Days.MONDAY);
        System.out.println("EnumSet Using add(): " + days1);

        // Using addAll() method
        days2.addAll(days1);
        System.out.println("EnumSet Using addAll(): " + days2);
 }
}

Thus, to the first EnumSet MONDAY gets added and in the next EnumSet using addAll() method all days get added.

This is the output for the preceding code.
add and addAll

EnumSet Iteration

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.

The code for accessing the enum set elements is this.

EnumSetExample.java

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

public class EnumSetExample {

    enum Days {
        MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
    }

    public static void main(String[] args) {

        EnumSet days = EnumSet.allOf(Days.class);
    Iterator iterate = days.iterator();

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

The output on running the code in IntelliJ is this.

access enumset

Removing elements of EnumSet

There are two methods to remove elements from an enum set.
The remove()method removes the specified element from the enum set.
The removeAll() which removes all the elements from the enum set.

This is the code for removing elements from an enum set.

EnumSetExample.java

import java.util.EnumSet;

public class EnumSetExample {

    enum Days {
        MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
    }

    public static void main(String[] args) {

        EnumSet days = EnumSet.allOf(Days.class);
       
       // Using remove()
        boolean value1 = days.remove(Days.FRIDAY);
        System.out.println("Is FRIDAY removed? " + value1);

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

Here, the remove() method removes FRIDAY and then later on the removeAll() method removes all days from the enum set, therefore the value2 will be set to true.

The output on running the code in IntelliJ is this.
remove and removeAll

Summary

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.

About SFG Contributor

Staff writer account for Spring Framework Guru

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