Java Enums

Learning Objectives

After completing this lesson, you will be able to:

• Understand what Enums are
• Create Enum constants
• Use Enums in switch statements
• Add fields and methods to Enums
• Create Enum constructors
• Override methods in Enums
• Iterate over Enum values
• Use EnumSet
• Use EnumMap
• Compare Enums
• Understand Enum internals
• Learn best practices
• Prepare for Java interview questions

Introduction

In many applications, certain values are fixed and should never change.

Examples:

• Days of the week
• Months
• Order Status
• Payment Status
• User Roles
• HTTP Methods
• Directions
• Database Types

Instead of using Strings:

String status = "ACTIVE";

Someone might accidentally write:

status = "Actve";

This typo compiles successfully but causes runtime bugs.

Enums solve this problem.

What is an Enum?

An Enum (Enumeration) is a special Java type used to define a fixed set of constants.

Example:

public enum Day {

    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY

}

Each value is an object of the Day enum.

Why Use Enums?

Without Enum

String role = "ADMIN";

Problems

• Typing mistakes
• Invalid values
• Difficult validation
• Less readable

With Enum

Role role = Role.ADMIN;

Benefits

• Type Safe
• Compile-time checking
• Better readability
• Cleaner code
• Easier maintenance

Enum Syntax

public enum Status {

    NEW,

    PROCESSING,

    COMPLETED,

    FAILED

}

Usage

Status status = Status.NEW;

System.out.println(status);

Output

NEW

Enum Constants

Each constant is actually a singleton object.

Status.NEW

Status.COMPLETED

Internally

Status

↓

NEW

↓

PROCESSING

↓

FAILED

Only one object exists for each constant.

Using Enums

public class Main {

    public static void main(String[] args) {

        Status status = Status.COMPLETED;

        System.out.println(status);

    }

}

Output

COMPLETED

Enum in Switch Statement

Status status = Status.PROCESSING;

switch(status){

    case NEW ->
            System.out.println("New Order");

    case PROCESSING ->
            System.out.println("Processing");

    case COMPLETED ->
            System.out.println("Completed");

    case FAILED ->
            System.out.println("Failed");

}

Output

Processing

Enums work naturally with switch expressions.

Enum Methods

Every Enum automatically inherits methods from java.lang.Enum.

Common methods

values()

valueOf()

name()

ordinal()

compareTo()

values()

Returns all constants.

for(Status status : Status.values()){

    System.out.println(status);

}

Output

NEW

PROCESSING

COMPLETED

FAILED

valueOf()

Converts String into Enum.

Status status =

Status.valueOf("NEW");

System.out.println(status);

Output

NEW

Throws IllegalArgumentException if the string does not exactly match an enum constant.

name()

Returns Enum name.

System.out.println(Status.NEW.name());

Output

NEW

ordinal()

Returns index position.

System.out.println(Status.COMPLETED.ordinal());

Output

2

Avoid using ordinal() for business logic because changing the declaration order changes the value.

Enum Comparison

Enums should be compared using ==.

if(status == Status.NEW){

    System.out.println("New Order");

}

Since Enum constants are singleton instances, == is safe and recommended.

Enum Constructors

Enums can have constructors.

public enum Status{

    NEW("Order Created"),

    COMPLETED("Order Delivered");

    private final String message;

    Status(String message){

        this.message = message;

    }

    public String getMessage(){

        return message;

    }

}

Usage

System.out.println(

Status.NEW.getMessage()

);

Output

Order Created

Enum constructors are implicitly private.

Enum Fields

Enums can store data.

public enum Planet{

    EARTH(9.8),

    MOON(1.62);

    private final double gravity;

    Planet(double gravity){

        this.gravity = gravity;

    }

    public double getGravity(){

        return gravity;

    }

}

Enum Methods

Enums can contain methods.

public enum Direction{

    NORTH,

    SOUTH;

    public void move(){

        System.out.println(

        "Moving");

    }

}

Usage

Direction.NORTH.move();

Abstract Methods in Enums

Each constant can provide its own implementation.

public enum Operation{

    ADD{

        @Override
        public int apply(int a,int b){

            return a+b;

        }

    },

    SUBTRACT{

        @Override
        public int apply(int a,int b){

            return a-b;

        }

    };

    public abstract int apply(int a,int b);

}

Usage

System.out.println(

Operation.ADD.apply(10,20)

);

Output

30

EnumSet

EnumSet is a specialized high-performance Set implementation for Enum types.

EnumSet<Day> weekends =

EnumSet.of(

Day.SATURDAY,

Day.SUNDAY

);

System.out.println(weekends);

Output

[SATURDAY, SUNDAY]

Benefits

• Faster than HashSet
• Memory efficient
• Type safe

EnumMap

EnumMap is a specialized Map implementation for Enum keys.

EnumMap<Day,String> schedule =

new EnumMap<>(Day.class);

schedule.put(

Day.MONDAY,

"Office"

);

schedule.put(

Day.SUNDAY,

"Holiday"

);

System.out.println(schedule);

Enum Internals

The compiler transforms an enum into a final class that extends java.lang.Enum.

Conceptually:

public final class Status

extends Enum<Status>

Each constant becomes a static final instance.

Status.NEW

↓

static final object

This is why Enum objects are immutable and singleton.

Enum Memory Diagram

               JVM Heap
                   │
        ┌──────────┼──────────┐
        │          │          │
      NEW     PROCESSING   COMPLETED
        │          │          │
     Singleton  Singleton  Singleton

Real-World Example: Order Status

public enum OrderStatus{

    PLACED,

    CONFIRMED,

    SHIPPED,

    DELIVERED,

    CANCELLED

}

Spring Boot applications frequently use enums for entity fields.

Real-World Example: User Roles

public enum Role{

    ADMIN,

    MANAGER,

    USER,

    GUEST

}

Real-World Example: Payment Status

public enum PaymentStatus{

    PENDING,

    SUCCESS,

    FAILED,

    REFUNDED

}

Best Practices

Use Enums Instead of String Constants

Prefer

Role.ADMIN

Instead of

"ADMIN"

Keep Enums Immutable

Declare fields as private final.

Avoid Using ordinal()

Use explicit fields if persistence or business logic requires stable numeric values.

Use EnumSet

Prefer EnumSet over HashSet for enum collections.

Use EnumMap

Prefer EnumMap over HashMap when keys are enums.

Common Mistakes

Comparing with equals()

Prefer

status == Status.NEW

Using ordinal() in Database

Changing enum order changes ordinal values.

Instead

NEW(1),

PROCESSING(2)

Store explicit codes if needed.

Using Strings Instead of Enums

Enums provide compile-time safety.

Creating Mutable Fields

Enum fields should generally be immutable.

Using valueOf() Without Validation

Invalid input throws IllegalArgumentException.

Handle user input safely before converting.

Hands-on Exercise

Create a Java program that:

1. Creates a Day enum.
2. Uses switch with enums.
3. Adds fields and constructors to an enum.
4. Creates methods inside an enum.
5. Implements abstract methods for enum constants.
6. Uses values().
7. Uses valueOf().
8. Uses EnumSet.
9. Uses EnumMap.
10. Builds a simple Order Management System using enums.

Summary

Enums provide a type-safe way to represent fixed sets of constants. Unlike string literals, enums prevent invalid values at compile time, improve readability, and integrate seamlessly with switch statements, collections, and enterprise frameworks. Java enums are full-fledged classes that can contain fields, constructors, methods, and even abstract methods. Specialized collections such as EnumSet and EnumMap make enum-based programming both efficient and expressive.

Key Takeaways

• Enums represent a fixed set of constants.
• Enum constants are singleton objects.
• Enums are type-safe.
• Enums can contain fields, constructors, and methods.
• Enum constructors are implicitly private.
• Use == to compare enum constants.
• EnumSet is optimized for enum collections.
• EnumMap is optimized for enum keys.
• Avoid using ordinal() for business logic.
• Enums are heavily used in Spring Boot, JPA, and enterprise applications.

Professional Interview Questions