How to create enum in java / Everything about enums in java

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What is an enum ?

An enum is a kind of data type in java which can contain a fixed set of values. These values are constants. The enum can have any one value out of these fixed set of values.

Enum Syntax

An enum is created using keyword enum followed by a user defined name and some fixed set of values enclosed between { and }.

Where is enum used

An enum is utilized when a particular type can have multiple fixed set of constant values. For Example, your application can have different types of user roles. Instead of using constants for these role types, an enum can be used as :

public enum ROLES {
     ADMIN, MANAGER, USER;
}

Other examples where enum can be used are when defining days of week, months of year, math operators, seasons, planets of solar system etc.

Enum constants are accessed using dot(.) operator along with name of enum type just like static values are accessed using a class name such as ROLES.ADMIN, ROLES.USER etc.
Enum constants can be assigned to variables which are of the same enum type such as

ROLES role = ROLES.ADMIN;

Since enum constants can be assigned to values of same enum type, they can be easily utilized in switch-case and if-else constructs as shown below

Every enum constant is of the same type as the enum.

 

How to use enum in switch statement

Below code shows how an enum can be used in a switch statement. case statements should be the constants of the enum and value in switch should be one of the constant of enum. The case statement which matches the enum value in switch is executed.

public class DemonstrateEnum {

   enum SEASON {
	SUMMER, WINTER, AUTUMN, SPRING;
   }

   public static void main(String[] args) {
	SEASON season = SEASON.AUTUMN;
	switch (season) {
	     case SUMMER: {
		System.out.println("Summer season");
		break;
	     }
	     case WINTER: {
		System.out.println("Winter season");
		break;
	     }
	     case AUTUMN: {
		System.out.println("Autumn season");
		break;
	     }
	     case SPRING: {
		System.out.println("Spring season");
		break;
	     }
	}
   }
}

enum can be defined inside the class and outside the class as well. When defined inside the class, it can be accessed by other classes using the class name in which it is defined such as DemonstrateEnum.SEASON season. When defined outside the class, enum can be accessed directly by its name.

If an enum is defined outside the class, it should not have public access. If it is assigned public access modifier, it should be defined in its own file(similar to a class).

How to use enum in if-else

Similar to switch-case statements, an enum can also be used in if-else statements. Below code compares the value of an enum constant with all the constants of the enum. Since both the values are constants, they are compared directly by == operator.

public class DemonstrateEnum {

   enum SEASON {
	SUMMER, WINTER, AUTUMN, SPRING;
   }

   public static void main(String[] args) {
	SEASON season = SEASON.AUTUMN;
	if(season == SEASON.SUMMER) {
	      System.out.println("Summer season");
	} else if(season == SEASON.WINTER) {
	      System.out.println("Winter season");
	} else if(season == SEASON.AUTUMN) {
	      System.out.println("Autumn season");
	} else if(season == SEASON.SPRING) {
	      System.out.println("Spring season");
	}
   }
}

How to iterate over Enum

Enum has a values method. This method when called on enum returns the array of enum values which can be iterated just like a normal array using a loop as shown below

SEASON[] seasons = SEASON.values();
for (int i = 0; i < seasons.length; i++) {
	SEASON season = seasons[i];
	System.out.println(season.name());
}

The above code produces following output
SUMMER
WINTER
AUTUMN
SPRING

Constructors, fields and methods in enum

An enum can contain constructors, fields and methods just like a normal class. This constructor is automatically called when enum constants are declared. Remember you cannot call enum constructor yourself, it is called on its own. An enum can also contain methods if required.
Below code declares an enum along with a field, constructor which initializes this field and a method which returns the value of this field.

public enum Season {

    SUMMER("summer"), AUTUMN("autumn"), WINTER("winter"), SPRING("spring");

  /**
     * enum field
     */
    private String seasonStr;

  /**
     * Constructor
     * @param s
     */
    private Season(String s) {
	seasonStr = s;
    }

  /**
     * Method which returns the season string
     * @return
     */
    public String getSeason(){
	return seasonStr;
    }
	
    public static void main(String[] args) {
	Season[] seasons = Season.values();
	for (Season season : seasons) {
		// access the enum field using its method
		System.out.println(season.getSeason());
	}
    }	
}
enum constructor can be assigned only default or private access modifier.

Let's tweak in

  1. If an enum contains fields and methods, the list of constants should end with a semi-colon(;).
  2. Enum constants should be declared at the very first line after enum declaration before any field, method or constructor declaration otherwise a compiler error will be generated.
  3. It is not mandatory to declare enum constants in upper case but it is considered as a standard.
  4. An enum cannot extend another enum. This is because every enum extends java.lang.Enum class and since java does not support multiple inheritance, it cannot extend any other class.

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