Enumerations are used to represent a set of named constants in programming languages. For example, the 4 suits in a deck of playing cards may be 4 enumerators named Club, Diamond, Heart, and Spade, which belong to the enumeration type named Suit. Other examples include natural enumerated types (such as planets, days of the week, colors, directions, etc.). Enumerations are used when we know all possible values ??at compile time, such as menu selections, rounding modes, command line flags, etc. The set of constants in an enumeration type does not need to remain fixed all the time.

In Java (since 1.5), enumerations are represented using the enum data type. Java enumerations are more powerful than C/C++ enumerations. In Java, we can also add variables, methods, and constructors to it. The main purpose of enum is to define our own data types (Enumerated Data Types).

Declaration of enum in 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

• First line inside enum should be list of constants then other things like methods, variables and constructor.
• According to Java naming conventions, it's recommended that we name constant with all capital letters

Important points of enum :

Every enum internally implemented by using Class.

/* internally above enum Color is converted to
class Color
{
     public static final Color RED = new Color();
     public static final Color BLUE = new Color();
     public static final Color GREEN = new Color();
}*/

• Every enum constant represents an object of type enum.
• enum type can be passed as an argument to switch statement.

// 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.

• Every enum constant is usually implicitly public static final. Since it's static, we will access it by using enum Name. Since it's final, we can’t create child enums.
• We can declare main() method inside enum. Hence we will invoke enum directly from the prompt .

// 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

Enum and Inheritance :

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

values(), ordinal() and valueOf() methods :

• These methods are present inside java.lang.Enum.
• values() method are often wont to return all values present inside enum.
• Order is vital in enums.By using ordinal() method, each enum constant index are often found, a bit like array index.
• valueOf() method returns the enum constant of the required string value, if exists

// 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

enum and constructor :

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