switch on strings and enums

Strings

Since Java 7, a switch can choose by a String. The labels are String literals, and a label matches when the String equals it, exactly: capitals count, and so do spaces. A radio reads commands:

Scanner scanner = new Scanner(System.in);
int volume = 5;
while (true) {
    String command = scanner.nextLine();
    if (command.equals("off")) {
        break;
    }
    switch (command) {
        case "louder" -> volume++;
        case "quieter" -> volume--;
        case "status" -> System.out.println("Volume " + volume);
        default -> System.out.println("Unknown command: " + command);
    }
}
System.out.println("Radio off at volume " + volume);

louder
louder
status
Quieter
off

Volume 7
Unknown command: Quieter
Radio off at volume 7

Quieter with a capital Q isn't quieter, so it goes to default. A program that should take both can switch on command.toLowerCase().

The check for off stays an if before the switch, and here's why: inside a switch, break ends the switch, not the loop around it. A break in a case would leave the switch and go on reading commands.

One more thing to know: a switch on a String that is null throws a NullPointerException, as calling a method on it would.

Enums

A switch on an enum (module 39) uses the constants' names as labels. A cinema's tickets come in three kinds:

public class Main {
    public static void main(String[] args) {
        System.out.println(price(Ticket.CHILD));
        System.out.println(price(Ticket.ADULT) + price(Ticket.SENIOR));
    }

    public static int price(Ticket ticket) {
        return switch (ticket) {
            case CHILD -> 6;
            case ADULT -> 12;
            case SENIOR -> 8;
        };
    }
}

public enum Ticket {
    CHILD, ADULT, SENIOR
}

6
20

• A label is the constant's name alone, case CHILD, not case Ticket.CHILD: the switch already knows the enum. (Since Java 21 the long name is allowed too, but the short one is the custom.)
• This switch expression has no default, and it doesn't need one: an enum has only the values it lists, and the three cases cover them all, so javac knows the switch always gives a value.

When the enum grows

Leaving out default pays off when someone adds a constant. Say the cinema starts selling STUDENT tickets:

public class Main {
    public static void main(String[] args) {
        System.out.println(price(Ticket.STUDENT));
    }

    public static int price(Ticket ticket) {
        return switch (ticket) {
            case CHILD -> 6;
            case ADULT -> 12;
            case SENIOR -> 8;
        };
    }
}

public enum Ticket {
    CHILD, ADULT, SENIOR, STUDENT
}

Main.java:7: error: the switch expression does not cover all possible input values
        return switch (ticket) {
               ^
1 error

javac points at every switch expression that has no case for STUDENT, and once each has one, the program is complete again. With default -> 0; in the switch, it would have compiled, and every student would have got in for free without anyone noticing. So for an enum, list every constant and leave default out.

A switch statement on an enum, with arrows or with colons, may leave constants out, like a switch statement on an int.