تعبيرات switch (Java الحديثة)

Written Explanation

The arrow form

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

Differences from the classic form:

• Each label is followed by -> and then one action: an expression, a block in braces, or a throw (which you will learn later).
• Only that action runs. There is no fall-through and no break.
• Several constants can share a rule with commas: case 6, 7 ->. You no longer stack empty labels.

You cannot mix the two styles in one switch. A case 1 -> next to a case 2: is rejected by the compiler (Common Mistakes).

The switch expression

A switch can be used as an expression, meaning it has a value, so it can sit on the right side of an assignment:

String type = switch (day) {
    case 6, 7 -> "Weekend";
    default -> "Weekday";
};

Notice the semicolon after the closing brace: this is one statement, String type = ...;. Each rule's right-hand side is the value for that case, and all values must be compatible with the type you assign to.

It must cover every case

A statement may do nothing if no label matches. An expression cannot: it must always produce a value. So the compiler requires a switch expression to be exhaustive: every possible selector value must be handled. For an int or a String that means a default, because there are far too many values to list. If you forget it, the compiler says so (Example 3). With enums and sealed types, which come later, you can be exhaustive without a default by listing all the possibilities; a later module uses this.

This requirement is a safety feature: you can no longer forget the "else" case, and the variable always gets a value.

Several statements: blocks and yield

Usually a rule is one expression. When a case needs more steps, use a block and end it with yield, which hands the value to the expression:

String plan = switch (day) {
    case 1, 2, 3, 4, 5 -> {
        int hours = 8;
        yield "Work " + hours + "h";
    }
    case 6 -> "Half day";
    default -> "Off";
};

yield is like return for the switch: it ends the block and supplies the value. Keep blocks short; if one grows large, the logic probably belongs somewhere else.

The arrow form as a statement

You can use the arrow form without producing a value, as in the first snippet. It is a statement and does not have to be exhaustive. It is the modern replacement for the classic switch in many codebases because there is no break to forget.

Version note

Switch expressions were previewed in JDK 12 and 13 and became a standard feature in Java 14 (JEP 361). Any JDK from version 14 onward supports them, including the one used for this course's examples (JDK 21). If you ever work in a project that still compiles with Java 8 or 11, only the classic form exists there; check the project's Java version before using the arrow form.