Enum Types in Java

In Java, ENUM is not only a list of constants — it is a full-featured data type.

Inside an enum, we can create different types of members, just like in a class.

Below are all the Enum Types (what an enum can contain or what kinds of enums exist).

1. Simple Enum (Basic Enum Type)

A simple enum contains only constant values.

Example:

enum Color {
    RED, GREEN, BLUE
}

• Most commonly used
• Constants only
• No fields or methods

2. Enum with Fields (Stateful Enum)

This enum stores extra information for each constant.

Example:

enum Status {
    SUCCESS(200),
    ERROR(500);

    private int code;

    Status(int code) {
        this.code = code;
    }

    public int getCode() {
        return code;
    }
}

• Each constant has its own value/state
• Uses constructor + fields

3. Enum with Methods

Enums can have custom methods.

Example:

enum Level {
    LOW, MEDIUM, HIGH;

    public String message() {
        return "Level is: " + this.name();
    }
}

• Methods behave like class methods
• Helps add behavior to constants

4. Enum with Abstract Methods (Constant-Specific Behavior)

Each enum constant can override methods (like polymorphism).

Example:

enum Operation {
    ADD {
        public int apply(int a, int b) { return a + b; }
    },
    SUBTRACT {
        public int apply(int a, int b) { return a - b; }
    };

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

• Constants behave differently
• Perfect for strategy patterns

5. Enum with Constructor (Private Only)

Enums can have private constructors.

—> You cannot call new with enums.

Example:

enum Size {
    SMALL("S"),
    LARGE("L");

    private String code;

    Size(String code) {
        this.code = code;
    }
}

• Constructor is always private
• Used to initialize constant values

6. Enum Implementing Interfaces

Enums can implement 1 or more interfaces.

Example:

interface Printable {
    void print();
}

enum Shape implements Printable {
    CIRCLE, SQUARE;

    public void print() {
        System.out.println(this.name());
    }
}

• Enums cannot extend classes
• But can implement interfaces

7. Nested Enum (Enum Inside a Class)

An enum can be declared inside a class.

Example:

class Vehicle {
    enum Type {
        CAR, BIKE, TRUCK
    }
}

• Helps organize constants
• Used in large projects

8. Enum with Main Method (Self-Test Enum)

Enums can contain a main() method for testing.

Example:

enum Day {
    MONDAY, FRIDAY, SUNDAY;

    public static void main(String[] args) {
        for (Day d : Day.values()) {
            System.out.println(d);
        }
    }
}

• Useful for quick testing
• Enum acts like a runnable class

9. Enum Singleton (Most Important Type)

Enum can be used to create perfect Singleton.

Example:

enum DatabaseConnection {
    INSTANCE;
}

• Thread-safe
• Lazy-loaded
• Serialization-proof
• Best Singleton pattern

10. Enum with Multiple Fields and Methods

Very common in real applications.

Example:

enum HttpStatus {
    OK(200, "Success"),
    BAD_REQUEST(400, "Invalid Request");

    private int code;
    private String message;

    HttpStatus(int code, String message) {
        this.code = code;
        this.message = message;
    }

    public int code() { return code; }
    public String message() { return message; }
}

• Store multiple values
• Create real-world domain models

Summary of Enum Types in Java