Enums: a fixed set of values

Some values can only be one of a few: a traffic light is red, yellow or green, a cup of coffee is small, medium or large. Module 17's TrafficLight challenge kept its color in a String, private String color;, and compared it with equals. That works as long as every String is spelled exactly right, but nothing stops a typo:

String light = "Green";
if (light.equals("green")) {
    System.out.println("Go");
} else {
    System.out.println("Stop");
}

Stop

The capital G makes it another String, and neither javac nor the program notices anything wrong. An enum (short for enumerated type) is a type that lists every value it can have:

public enum Light {
    RED, YELLOW, GREEN
}

• It's written like a class, with the word enum in place of class, in a file of its own, Light.java.
• The values are called constants. They're separated by commas, and by custom written in capital letters.
• A variable of type Light can hold one of these three, and nothing else (or null, like any object variable).

public class Main {
    public static void main(String[] args) {
        Light light = Light.GREEN;
        System.out.println(light);
        if (light == Light.GREEN) {
            System.out.println("Go");
        }
        light = Light.RED;
        System.out.println(light == Light.GREEN);
    }
}

public enum Light {
    RED, YELLOW, GREEN
}

GREEN
Go
false

• A constant is written with the enum's name and a dot: Light.GREEN.
• Printing a constant prints its name.
• Constants are compared with ==. Each constant is one object, made once, the first time the program uses the enum, so there's never a second GREEN, and == is right here, unlike for Strings. (equals gives the same answers.)

A typo is now javac's problem, not the user's:

public class Main {
    public static void main(String[] args) {
        Light light = Light.GREN;
        System.out.println(light);
    }
}

public enum Light {
    RED, YELLOW, GREEN
}

Main.java:3: error: cannot find symbol
        Light light = Light.GREN;
                           ^
  symbol:   variable GREN
  location: class Light
1 error

values(), name() and ordinal()

• Light.values() returns an array of all the constants, in the order they're written in the enum. A for-each loop goes through it.
• name() returns a constant's name as a String, such as "RED".
• ordinal() returns a constant's position in the enum, counted from 0.

public class Main {
    public static void main(String[] args) {
        for (Light light : Light.values()) {
            System.out.println(light.ordinal() + ": " + light.name().toLowerCase());
        }
        System.out.println(Light.values().length + " lights");
    }
}

public enum Light {
    RED, YELLOW, GREEN
}

0: red
1: yellow
2: green
3 lights

An enum is a type

An enum can be the type of a variable, a parameter, a return value, an instance variable or a list's elements, like any class. A method can check which constant it got:

public class Main {
    public static void main(String[] args) {
        System.out.println(canCross(Light.GREEN));
        System.out.println(canCross(Light.YELLOW));
    }

    public static boolean canCross(Light light) {
        return light == Light.GREEN;
    }
}

public enum Light {
    RED, YELLOW, GREEN
}

true
false

Java also has Light.valueOf("RED"), which gives the constant with that name. For a text that isn't a constant's name, it stops the program with an error, and part 11 covers handling errors like that.