enum in Java

In Java, Enumerations or Java Enum serve the purpose of representing a group of named constants in a programming language. Java Enums are used when we know all possible values at compile time, such as choices on a menu, rounding modes, command-line flags, etc. The set of constants in an enum type doesn’t need to stay fixed for all time.

What is Enumeration or Enum in Java?

A Java enumeration is a class type. Although we don’t need to instantiate an enum using new, it has the same capabilities as other classes. This fact makes Java enumeration a very powerful tool. Just like classes, you can give them constructors, add instance variables and methods, and even implement interfaces.

Enum Example:

The 4 suits in a deck of playing cards may be 4 enumerators named Club, Diamond, Heart, and Spade, belonging to an enumerated type named Suit. Other examples include natural enumerated types (like the planets, days of the week, colors, directions, etc.). 

One thing to keep in mind is that, unlike classes, enumerations neither inherit other classes nor can get extended(i.e become superclass).  We can also add variables, methods, and constructors to it. The main objective of an enum is to define our own data types(Enumerated Data Types).

Declaration of enum in Java

Enum declaration can be done outside a Class or inside a Class but not inside a Method.

1. Declaration outside the class

Java

// A simple enum example where enum is declared
// outside any class (Note enum keyword instead of
// class keyword)

enum Color {
    RED,
    GREEN,
    BLUE;
}

public class Test {
    // Driver method
    public static void main(String[] args)
    {
        Color c1 = Color.RED;
        System.out.println(c1);
    }
}

Output

RED

2. Declaration inside a class

Java

// enum declaration inside a class.

public class Test {
    enum Color {
        RED,
        GREEN,
        BLUE;
    }

    // Driver method
    public static void main(String[] args)
    {
        Color c1 = Color.RED;
        System.out.println(c1);
    }
}

Output

RED

• The first line inside the enum should be a list of constants and then other things like methods, variables, and constructors.
• According to Java naming conventions, it is recommended that we name constant with all capital letters

Properties of Enum in Java

There are certain properties followed by Enum as mentioned below:

• Every enum is internally implemented by using Class.
• Every enum constant represents an object of type enum.
• Enum type can be passed as an argument to switch statements.
• Every enum constant is always implicitly public static final. Since it is static, we can access it by using the enum Name. Since it is final, we can’t create child enums.
• We can declare the main() method inside the enum. Hence we can invoke the enum directly from the Command Prompt.

Below is the implementation of the above properties:

Java

// A Java program to demonstrate working on enum
// in switch case (Filename Test. Java)

import java.util.Scanner;

// An Enum class
enum Day {
    SUNDAY,
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY;
}

// Driver class that contains an object of "day" and
// main().
public class Test {
    Day day;

    // Constructor
    public Test(Day day) { this.day = day; }

    // Prints a line about Day using switch
    public void dayIsLike()
    {
        switch (day) {
        case MONDAY:
            System.out.println("Mondays are bad.");
            break;
        case FRIDAY:
            System.out.println("Fridays are better.");
            break;
        case SATURDAY:
        case SUNDAY:
            System.out.println("Weekends are best.");
            break;
        default:
            System.out.println("Midweek days are so-so.");
            break;
        }
    }

    // Driver method
    public static void main(String[] args)
    {
        String str = "MONDAY";
        Test t1 = new Test(Day.valueOf(str));
        t1.dayIsLike();
    }
}

Output

Mondays are bad.

Java Enum Programs

1. Main Function Inside Enum

We can declare a main function inside an enum as we can invoke the enum directly from the Command Prompt.

Below is the implementation of the above property:

Java

// A Java program to demonstrate that we can have
// main() inside enum class.

enum Color {
    RED,
    GREEN,
    BLUE;

    // Driver method
    public static void main(String[] args)
    {
        Color c1 = Color.RED;
        System.out.println(c1);
    }
}

Output

RED

2. Loop through Enum

We can iterate over the Enum using values( ) and loop. values() function returns an array of Enum values as constants using which we can iterate over the values.

Below is the implementation of the loop through Enum:

Java

// Java Program to Print all the values
// inside the enum using for loop
import java.io.*;

// Enum Declared
enum Color {
    RED,
    GREEN,
    BLUE;
}

// Driver Class
class GFG {

    // Main Function
    public static void main(String[] args)
    {
        // Iterating over all the values in
        // enum using for loop
        for (Color var_1 : Color.values()) {
            System.out.println(var_1);
        }
    }
}

Output

RED
GREEN
BLUE

3. Enum in a Switch Statement

Java

// Java Program to implement
// Enum in a Switch Statement
import java.io.*;

// Driver Class
class GFG {
    // Enum Declared
    enum Color {
        RED,
        GREEN,
        BLUE,
          Yellow;
    }

    // Main Function
    public static void main(String[] args)
    {
          Color var_1=Color.Yellow;

          // Switch case with Enum
          switch(var_1){
          case RED:
            System.out.println("Red color observed");
            break;
          case GREEN:
            System.out.println("Green color observed");
            break;
          case BLUE:
            System.out.println("Blue color observed");
          default:
            System.out.println("Other color observed");
        }
    }
}

Output

Other color observed

Enum and Inheritance

• All enums implicitly extend java.lang.Enum class. As a class can only extend one parent in Java, an enum cannot extend anything else.
• toString() method is overridden in java.lang.Enum class, which returns enum constant name.
• enum can implement many interfaces.

Enum and Constructor

• Enum can contain a constructor and it is executed separately for each enum constant at the time of the enum class loading.
• We can’t create enum objects explicitly and hence we can’t invoke the enum constructor directly.

Enum and Methods

Enum can contain both concrete methods and abstract methods. If an enum class has an abstract method, then each instance of the enum class must implement it.

Java

// Java program to demonstrate that enums can have
// constructor and concrete methods.

// An enum (Note enum keyword inplace of class keyword)
enum Color {
    RED,
    GREEN,
    BLUE;

    // enum constructor called separately for each
    // constant
    private Color()
    {
        System.out.println("Constructor called for : "
                           + this.toString());
    }

    public void colorInfo()
    {
        System.out.println("Universal Color");
    }
}

public class Test {
    // Driver method
    public static void main(String[] args)
    {
        Color c1 = Color.RED;
        System.out.println(c1);
        c1.colorInfo();
    }
}

Output

Constructor called for : RED
Constructor called for : GREEN
Constructor called for : BLUE
RED
Universal Color

FAQs on Enum in Java

Q1. Can we create the instance of Enum by the new keyword?

Ans:

No, we can’t create the instance of the Enum keyword because it contains private constructors only.

Q2. Can we have an abstract method in the Enum?

Ans:

Yes, we have an abstract method in Enum.

Q3. What is the purpose of the values() method in the enum?

Ans:

In Java, the values( ) method can be used to return all values present inside the enum.

Q4. What is the purpose of the valueOf() method in the enum?

Ans:

The valueOf() method returns the enum constant of the specified string value if exists.

Q5. What is the purpose of the ordinal() method in the enum?

Ans:

By using the ordinal() method, each enum constant index can be found, just like an array index.

Q6.  Write a program in Java to describe the use of values(), valueOf(), and ordinal() methods in the enum.

Ans:

Java

// Java program to demonstrate working of values(),
// ordinal() and valueOf()

enum Color {
    RED,
    GREEN,
    BLUE;
}

public class Test {
    public static void main(String[] args)
    {
        // Calling values()
        Color arr[] = Color.values();

        // enum with loop
        for (Color col : arr) {
            // Calling ordinal() to find index
            // of color.
            System.out.println(col + " at index "
                               + col.ordinal());
        }

        // Using valueOf(). Returns an object of
        // Color with given constant.
        // Uncommenting second line causes exception
        // IllegalArgumentException
        System.out.println(Color.valueOf("RED"));
        // System.out.println(Color.valueOf("WHITE"));
    }
}

Output

RED at index 0
GREEN at index 1
BLUE at index 2
RED

Next Article on enum: Enum with Customized Value in Java

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