In Java language, enumeration is commonly referred to as enums. Enums are special data types in Java that allow for the definition of a set of named constants. They are very useful when you have a fixed set of related constants that make your code more readable and less error-prone. This article explains the basics of enums in Java and their usage and provides examples to illustrate their functionality.
What is enum?
In the Java language, an enum is a special data type that represents a group of constants(unchangeable variables, like final variables). It is used to define a collection of named constants that can be assigned to variables. Enums are very useful when you have a fixed set of related constants, such as days of the week, direction state, etc.
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Declaration of enum in Java
In this section, we will define an enum in Java outside the World class. The following program demonstrates the enum in Java language:
Program
enum Country {
CANADA,
JAPAN,
BRAZIL,
GERMANY,
INDIA
}
public class Main {
public static void main(String[] args) {
for (Country country : Country.values()) {
System.out.println(country);
}
}
}
Output
CANADA
JAPAN
BRAZIL
GERMANY
INDIA
Declaring Enum class Inside the Class
We can also define the enum inside the class in Java. In this case, the scope of the enum is limited to that class and is used when the enum is closely related to the class and not intended to be used outside of it.
The following program demonstrates the declaring enum:
Program
public class Main {
enum Country {
CANADA,
JAPAN,
BRAZIL,
GERMANY,
INDIA
}
public static void main(String[] args) {
for (Country country : Country.values()) {
System.out.println(country);
}
}
}
Output
CANADA
JAPAN
BRAZIL
GERMANY
INDIA
Enum Inside a Class
Enum can be declared within a class, just like a nested class. This encapsulates the enum within the class, making it more organized and modular. It is a common practice when the enum is closely associated with the class, enhancing readability and maintainability.
The following program demonstrates the enum class inside a Main class:
Program
public class Season {
enum Month {
JANUARY("Winter"), FEBRUARY("Winter"), MARCH("Spring"),
APRIL("Spring"), MAY("Spring"), JUNE("Summer"),
JULY("Summer"), AUGUST("Summer"), SEPTEMBER("Fall"),
OCTOBER("Fall"), NOVEMBER("Fall"), DECEMBER("Winter");
private final String season;
Month(String season) {
this.season = season;
}
public String getSeason() {
return season;
}
}
private Month currentMonth;
public Season(Month currentMonth) {
this.currentMonth = currentMonth;
}
public String getMonthInfo() {
return currentMonth.name() + " is in the season of " + currentMonth.getSeason();
}
public void nextMonth() {
int currentIndex = currentMonth.ordinal();
Month[] months = Month.values();
currentMonth = months[(currentIndex + 1) % months.length];
}
public static void main(String[] args) {
Season season = new Season(Month.JANUARY);
System.out.println(season.getMonthInfo());
season.nextMonth();
System.out.println(season.getMonthInfo());
season.nextMonth();
System.out.println(season.getMonthInfo());
}
}
Output
JANUARY is in the season of Winter
FEBRUARY is in the season of Winter
MARCH is in the season of Spring
Enum in a Switch Statement
Java Developers can also use enum with switch statements. It allows for clear and readable control flow based on the enum values. Let us take an example of a Month Example.
The following program demonstrates the switch statement with an enum:
Program
public class Season {
// Enum declared inside the Season class
enum Month {
JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE,
JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER
}
public static void printSeason(Month month) {
switch (month) {
case DECEMBER:
case JANUARY:
case FEBRUARY:
System.out.println(month + " is in Winter.");
break;
case MARCH:
case APRIL:
case MAY:
System.out.println(month + " is in Spring.");
break;
case JUNE:
case JULY:
case AUGUST:
System.out.println(month + " is in Summer.");
break;
case SEPTEMBER:
case OCTOBER:
case NOVEMBER:
System.out.println(month + " is in Fall.");
break;
default:
System.out.println("Unknown month.");
break;
}
}
public static void main(String[] args) {
printSeason(Month.JANUARY);
printSeason(Month.APRIL);
printSeason(Month.AUGUST);
printSeason(Month.OCTOBER);
}
}
Output
JANUARY is in Winter.
APRIL is in Spring.
AUGUST is in Summer.
OCTOBER is in the Fall.
Loop Through an Enum
We can also iterate all elements in the enum class using loops. There are two ways to do this: we can use the normal for loop or the for each loop. In the example, we will iterate the enum element by using the loop.
The following program demonstrates the enum with a loop:
Program
public class Season {
// Enum declared inside the Season class
enum Month {
JANUARY("Winter"), FEBRUARY("Winter"), MARCH("Spring"),
APRIL("Spring"), MAY("Spring"), JUNE("Summer"),
JULY("Summer"), AUGUST("Summer"), SEPTEMBER("Fall"),
OCTOBER("Fall"), NOVEMBER("Fall"), DECEMBER("Winter");
private final String season;
Month(String season) {
this.season = season;
}
public String getSeason() {
return season;
}
}
public static void printAllSeasons() {
// Using a for loop to iterate over all the months in the enum
for (Month month : Month.values()) {
System.out.println(month.name() + " is in the season of " + month.getSeason());
}
}
public static void printAllSeasons2(){
Month[] month2 = Month.values() ;
for(int i = 0;i<month2.length ;i++){
System.out.println(month2[i]);
}
}
public static void main(String[] args) {
// Call the method to print the season for each month
System.out.println("First Loop");
printAllSeasons();
System.out.println("Second Loop");
System.out.println();
printAllSeasons2();
}
}
Output
First Loop
JANUARY is in the season of Winter
FEBRUARY is in the season of Winter
MARCH is in the season of Spring
APRIL is in the season of Spring
MAY is in the season of Spring
JUNE is in the season of Summer
JULY is in the season of Summer
AUGUST is in the season of Summer
SEPTEMBER is in the season of Fall
OCTOBER is in the season of Fall
NOVEMBER is in the season of Fall
DECEMBER is in the season of Winter
Second Loop
JANUARY
FEBRUARY
MARCH
APRIL
MAY
JUNE
JULY
AUGUST
SEPTEMBER
OCTOBER
NOVEMBER
DECEMBER
Main Function Inside Enum
In Java, enums can have a main function. This allows them to be invoked directly from the command line. Enums are typically used to represent a set of predefined constants and encapsulate their behavior. The following program demonstrates the enum:
Program
public enum Operation {
ADD((x, y) -> x + y),
SUBTRACT((x, y) -> x - y),
MULTIPLY((x, y) -> x * y),
DIVIDE((x, y) -> y != 0 ? x / y : handleDivideByZero());
private final OperationStrategy strategy;
Operation(OperationStrategy strategy) {
this.strategy = strategy;
}
public int apply(int x, int y) {
return strategy.apply(x, y);
}
private static int handleDivideByZero() {
System.out.println("Cannot divide by zero");
return 0;
}
private interface OperationStrategy {
int apply(int x, int y);
}
public static void main(String[] args) {
// Example usage of the enum and its apply method
int resultAdd = Operation.ADD.apply(19, 30);
int resultMultiply = Operation.MULTIPLY.apply(34, 72);
System.out.println("Result of ADD operation: " + resultAdd);
System.out.println("Result of MULTIPLY operation: " + resultMultiply);
}
}
Output
Result of ADD operation: 49
Result of MULTIPLY operation: 2448
Properties of Enum in Java
There are certain properties followed by an enum, as mentioned below:
Class Type: Every enum is internally implemented as the class type.
Enum Constants: Each enum constant represents an object of type enum.
Switch Statements: Enum types can be used in switch statements.
Implicit Modifiers: Every enum constant is implicitly public static final. It can be accessed using the enum name. It is final, and enums cannot be extended.
Main Method: Enums can declare a main() method. It allows direct invocation from the command line.
The following program demonstrates the implementation of the above properties:
Program
import java.util.Scanner;
enum Day {
SUNDAY,
MONDAY,
TUESDAY,
WEDNESDAY,
THURSDAY,
FRIDAY,
SATURDAY;
}
public class Main {
Day day;
// Constructor
public Main(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 in Java langauge
public static void main(String[] args) {
String str = "MONDAY";
Main ob = new Main(Day.valueOf(str));
ob.dayIsLike();
}
}
Output
Mondays are bad.
Difference Between Enums and Classes
Feature
Classes
Enums
Purpose
They represent a fixed set of constants
They used to define objects with attributes and methods
Inheritance
It cannot be extended by other classes
It can be extended by other classes.
State
The enum constants do not have a state
The Objects of a class can maintain state through fields/attributes
Instantiation
Enum instantiate during the new keyword.
Enum constants are created implicitly, and the new keyword cannot be used
Example
‘enum Day {MONDAY, TUESDAY, WEDNESDAY}
‘class Car {String model; int speed;}’
Enum and Inheritance
Enums Cannot extend Other classes: Enums in Java implicitly extend the ‘java.lang.Enum’ class, making the final. This means you cannot extend an enum or any other class.
Enums can implement Interfaces: Although enums cannot extend other classes, they can implement one or more interfaces. This allows them to inherit behavior in a controlled manner.
State Management: Enums implementing interfaces are useful in scenarios where different states or actions need to be defined and managed in a type-safe way.
Enum and Constructor
We can define a constructor inside an enum to initialize fields associated with each constant.
In Java, we cannot instantiate new enum objects with the new keyword as we do in the case of Java class objects.
Each object invokes an enum constructor with its required arguments.
Conclusion
Enumeration or enums in Java provide a powerful way to define a set of named constants, making your source code more readable, maintainable, and less error-prone. Enums are more than just simple constants; they can have fields, methods, and constructors. They allow you to associate behaviors and states with specific enum values. Their ability to integrate fields methods and even implement interfaces allows enums to go beyond mere symbolic constants, serving as a sophisticated tool in your programming.
FAQs
What is an enum in Java?
An enum in Java is a special data type that allows you to define a set of named constants. Enums are used to represent a group of predefined constants, such as days of the week, directions, or states.
Can an enum have methods and fields?
Yes, enums in Java can have methods, fields, and even constructors. This allows enums to hold additional data and behavior beyond just being constants.
Can an enum implement interfaces in Java?
Yes, an enum can implement interfaces. This allows you to enforce specific behavior across all constants in the enum.
Are enums type-safe in Java?
Yes, enums in Java are type-safe. It means you can’t assign any value other than the defined constants to an enum variable.
What is the purpose of the ‘valueOf()’ method in enums
The valueOf() method converts a string to the corresponding enum constant. If the string does not match any constant. It will throw an IllegalArgumentException.
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