20. Enums (Enumerations)

[!NOTE] Often, an application has a variable that should only ever be one of three or four specific things. A user's role might be Admin, Moderator, or Guest. If you represent this as a String, a developer might accidentally set a user's role to "Potato".

We solve this using Enums.

Defining an Enum

An enum is a special "class" that represents a group of constants (unchangeable variables).

// We declare all possible valid states globally
public enum Level {
  LOW,
  MEDIUM,
  HIGH
}

  

Now, when you build a class, you don't use a String for the difficulty variable. You strictly type it to the Level enum!

public class GameSession {
    // The type is 'Level', not 'String'!
    Level currentDifficulty;

    public GameSession() {
        // You cannot assign "SUPER_HARD". The compiler restricts you to the 3 constants.
        currentDifficulty = Level.MEDIUM; 
    }
}

  

Using Enums in Switch Statements

Enums shine the brightest when combined with a switch statement. It prevents developers from having to memorize random integer codes (like "Role 1 is Admin, Role 2 is Guest").

Level myVar = Level.HIGH;

switch(myVar) {
  case LOW:
    System.out.println("Low level");
    break;
  case MEDIUM:
     System.out.println("Medium level");
    break;
  case HIGH:
    System.out.println("High level executed!");
    break;
}

  

Advanced Enums

Because Enums in Java are effectively fully-featured classes behind the scenes, you can actually add instance variables, constructors, and methods directly inside the Enum definition itself!

public enum Role {
    // Define the enums and pass a parameter into their secret constructor!
    ADMIN(99),
    MODERATOR(50),
    GUEST(1);

    private int powerLevel;

    // Private constructor used ONLY internally by the Enum creation above
    private Role(int power) {
        this.powerLevel = power;
    }

    public int getPowerLevel() {
        return this.powerLevel;
    }
}

// Somewhere else in code:
System.out.println(Role.ADMIN.getPowerLevel()); // Outputs 99!

  

enum OrderStatus {
    CREATED,
    PAID,
    SHIPPED,
    CANCELLED;

    boolean canCancel() {
        return this == CREATED || this == PAID;
    }
}
  

OrderStatus status = OrderStatus.PAID;

if (status.canCancel()) {
    System.out.println("Order can still be cancelled");
}