Enums

TL;DR

• An Enum (Enumeration) is a special Java type used to define collections of constants (e.g., Days of the week, HTTP Status codes).
• Under the hood, an Enum is actually a fully-fledged Class.
• Enums provide type safety, exhaustive switch statement support, and built-in methods like values() and valueOf().

Concept

Before Java 5, developers used “Int Constants” or “String Constants” to represent fixed categories:
public static final int STATUS_PENDING = 1;
The problem was there was zero type safety. You could pass the number 99 into a setStatus(int) method, and the compiler wouldn’t care, causing a runtime crash.

Enums solve this by strictly restricting the possible values to a predefined list. If a method takes a Status, you must pass a valid Status enum.

Examples

public enum CoffeeSize {
    // These are actually public static final instances of CoffeeSize!
    SMALL(150), 
    MEDIUM(250), 
    LARGE(400); 

    // Enums can have fields, constructors, and methods
    private final int milliliters;

    // The constructor is implicitly PRIVATE. You cannot call 'new CoffeeSize()'.
    CoffeeSize(int milliliters) {
        this.milliliters = milliliters;
    }

    public int getMilliliters() {
        return milliliters;
    }
}

public class EnumDemo {
    public static void main(String[] args) {
        CoffeeSize mySize = CoffeeSize.MEDIUM;
        
        System.out.println("Size: " + mySize.getMilliliters() + "ml");
        
        // Built-in iteration
        for (CoffeeSize size : CoffeeSize.values()) {
            System.out.println(size);
        }
    }
}

Interview Questions

Q: Can an Enum extend another class?
A: No. Every enum implicitly extends java.lang.Enum. Since Java doesn’t support multiple inheritance, an enum cannot extend any other class. However, it can implement multiple interfaces!

Q: What is the difference between name() and toString() on an enum?
A: - name() is a final method defined in java.lang.Enum. It returns the exact string literal used to declare the enum constant (e.g., "MEDIUM"). It cannot be overridden.

• toString() also returns the name by default, but it is not final. You can override it to return a more user-friendly string (e.g., "Medium (250ml)"). You should always use name() for serialization/database storage, and toString() for UI display.