8 Switch Expressions

Like all expressions, switch expressions evaluate to a single value and can be used in statements. They may contain "case L ->" labels that eliminate the need for break statements to prevent fall through. You can use a yield statement to specify the value of a switch expression.

For background information about the design of switch expressions, see JEP 361.

"case L ->" Labels

Consider the following switch statement that prints the number of letters of a day of the week:

public enum Day { SUNDAY, MONDAY, TUESDAY,
    WEDNESDAY, THURSDAY, FRIDAY, SATURDAY; }

// ...

    int numLetters = 0;
    Day day = Day.WEDNESDAY;
    switch (day) {
        case MONDAY:
        case FRIDAY:
        case SUNDAY:
            numLetters = 6;
            break;
        case TUESDAY:
            numLetters = 7;
            break;
        case THURSDAY:
        case SATURDAY:
            numLetters = 8;
            break;
        case WEDNESDAY:
            numLetters = 9;
            break;
        default:
            throw new IllegalStateException("Invalid day: " + day);
    }
    System.out.println(numLetters);

It would be better if you could "return" the length of the day's name instead of storing it in the variable numLetters; you can do this with a switch expression. Furthermore, it would be better if you didn't need break statements to prevent fall through; they are laborious to write and easy to forget. You can do this with a new kind of case label. The following is a switch expression that uses the new kind of case label to print the number of letters of a day of the week:

    Day day = Day.WEDNESDAY;    
    System.out.println(
        switch (day) {
            case MONDAY, FRIDAY, SUNDAY -> 6;
            case TUESDAY                -> 7;
            case THURSDAY, SATURDAY     -> 8;
            case WEDNESDAY              -> 9;
            default -> throw new IllegalStateException("Invalid day: " + day);
        }
    );    

The new kind of case label has the following form:

case label_1, label_2, ..., label_n -> expression;|throw-statement;|block 

When the Java runtime matches any of the labels to the left of the arrow, it runs the code to the right of the arrow and does not fall through; it does not run any other code in the switch expression (or statement). If the code to the right of the arrow is an expression, then the value of that expression is the value of the switch expression.

You can use the new kind of case label in switch statements. The following is like the first example, except it uses "case L ->" labels instead of "case L:" labels:

    int numLetters = 0;
    Day day = Day.WEDNESDAY;
    switch (day) {
        case MONDAY, FRIDAY, SUNDAY -> numLetters = 6;
        case TUESDAY                -> numLetters = 7;
        case THURSDAY, SATURDAY     -> numLetters = 8;
        case WEDNESDAY              -> numLetters = 9;
        default -> throw new IllegalStateException("Invalid day: " + day);
    };
    System.out.println(numLetters);

A "case L ->" label along with its code to its right is called a switch labeled rule.

"case L:" Statements and the yield Statement

You can use "case L:" labels in switch expressions; a "case L:" label along with its code to the right is called a switch labeled statement group:

    Day day = Day.WEDNESDAY;
    int numLetters = switch (day) {
        case MONDAY:
        case FRIDAY:
        case SUNDAY:
            System.out.println(6);
            yield 6;
        case TUESDAY:
            System.out.println(7);
            yield 7;
        case THURSDAY:
        case SATURDAY:
            System.out.println(8);
            yield 8;
        case WEDNESDAY:
            System.out.println(9);
            yield 9;
        default:
            throw new IllegalStateException("Invalid day: " + day);
    };
    System.out.println(numLetters);

The previous example uses yield statements. They take one argument, which is the value that the case label produces in a switch expression.

The yield statement makes it easier for you to differentiate between switch statements and switch expressions. A switch statement, but not a switch expression, can be the target of a break statement. Conversely, a switch expression, but not a switch statement, can be the target of a yield statement.

Note:

It's recommended that you use "case L ->" labels. It's easy to forget to insert break or yield statements when using "case L:" labels; if you do, you might introduce unintentional fall through in your code.

For "case L ->" labels, to specify multiple statements or code that are not expressions or throw statements, enclose them in a block. Specify the value that the case label produces with the yield statement:

    int numLetters = switch (day) {
        case MONDAY, FRIDAY, SUNDAY -> {
            System.out.println(6);
            yield 6;
        }
        case TUESDAY -> {
            System.out.println(7);
            yield 7;
        }
        case THURSDAY, SATURDAY -> {
            System.out.println(8);
            yield 8;
        }
        case WEDNESDAY -> {
            System.out.println(9);
            yield 9;
        }
        default -> {
            throw new IllegalStateException("Invalid day: " + day);
        }
    };  

Exhaustiveness of switch Expressions

The cases of a switch expression must be exhaustive, which means that for all possible values, there must be a matching switch label. Thus, a switch expression normally requires a default clause. However, for an enum switch expression that covers all known constants, the compiler inserts an implicit default clause.

In addition, a switch expression must either complete normally with a value or complete abruptly by throwing an exception. For example, the following code doesn't compile because the switch labeled rule doesn't contain a yield statement:

int i = switch (day) {
    case MONDAY -> {
        System.out.println("Monday"); 
        // ERROR! Block doesn't contain a yield statement
    }
    default -> 1;
};

The following example doesn't compile because the switch labeled statement group doesn't contain a yield statement:

i = switch (day) {
    case MONDAY, TUESDAY, WEDNESDAY: 
        yield 0;
    default: 
        System.out.println("Second half of the week");
        // ERROR! Group doesn't contain a yield statement
};

Because a switch expression must evaluate to a single value (or throw an exception), you can't jump through a switch expression with a break, yield, return, or continue statement, like in the following example:

z: 
    for (int i = 0; i < MAX_VALUE; ++i) {
        int k = switch (e) { 
            case 0:  
                yield 1;
            case 1:
                yield 2;
            default: 
                continue z; 
                // ERROR! Illegal jump through a switch expression 
        };
    // ...
    }

Exhaustiveness of switch Statements

Note:

This feature is part of JEP 420, which is a preview feature.

The cases of a switch statement must be exhaustive if it uses pattern or null labels, or if its selector expression isn't one of the legacy types (char, byte, short, int, Character, Byte, Short, Integer, String, or an enum type).

The following example doesn't compile because the switch statement (which uses pattern labels) is not exhaustive:

    static void testSwitchStatementExhaustive(Object obj) {
        switch (obj) {  // error: the switch statement does not cover
                        //        all possible input values
            case String s: 
                System.out.println(s);
                break;
            case Integer i:
                System.out.println("Integer");
                break;
        }    
    }

You can make it exhaustive by adding a default clause:

    static void testSwitchStatementExhaustive(Object obj) {
        switch (obj) { 
            case String s: 
                System.out.println(s);
                break;
            case Integer i:
                System.out.println("Integer");
                break;
            default:
                break;
        }    
    }