6 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
Unlike switch statements, the cases of switch
expressions must be exhaustive, which means that for all possible values,
there must be a matching switch label. Thus, switch expressions
normally require a default clause. However, for
enum
switch expressions that cover 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
};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
};
// ...
}