switch

The classic form

switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 6:
    case 7:
        System.out.println("Weekend");
        break;
    default:
        System.out.println("Midweek");
}

Execution jumps to the matching label and continues until a break. That is fall-through, and it is why two labels stacked together work as an “or”.

It is also the classic bug. Omit a break and the next case’s body runs too, silently. Most of the switch statements in the wild that misbehave misbehave for this reason.

The arrow form

switch (day) {
    case 1 -> System.out.println("Monday");
    case 6, 7 -> System.out.println("Weekend");
    default -> System.out.println("Midweek");
}

No fall-through, no break, several labels on one line. Prefer this. It removes the failure mode rather than asking you to remember.

switch as an expression

The arrow form can produce a value:

String name = switch (day) {
    case 1 -> "Monday";
    case 6, 7 -> "Weekend";
    default -> "Midweek";
};

Note the semicolon — it is an expression, so the statement needs terminating.

As an expression, switch must be exhaustive: every possible value covered, or a default. The compiler enforces it, which is genuinely useful with enums, because adding a constant then breaks the build at every switch that has not been updated rather than silently taking the default.

For a multi-line branch use a block and yield:

case 6, 7 -> {
    log("weekend request");
    yield "Weekend";
}

What can be switched on

int and the smaller integral types, char, String, and enums. Not long, float or double.

With enums, omit the type name in the labels:

switch (status) {
    case ACTIVE -> ...;
    case CLOSED -> ...;
}

When not to use it

A switch over a type, where each branch does something the object could do itself, is usually a missed polymorphism. If adding a new case means editing several switches in different files, that is the signal.