Java Reference
In-Depth Information
break ; // Stop here
As you can see from the example, the various entry points into a switch statement
are labeled either with the keyword case , followed by an integer value and a colon,
or with the special default keyword, followed by a colon. When a switch statement
executes, the interpreter computes the value of the expression in parentheses and
then looks for a case label that matches that value. If it finds one, the interpreter
starts executing the block of code at the first statement following the case label. If it
does not find a case label with a matching value, the interpreter starts execution at
the first statement following a special-case default : label. Or, if there is no default :
label, the interpreter skips the body of the switch statement altogether.
Note the use of the break keyword at the end of each case in the previous code. The
break statement is described later in this chapter, but, in this case, it causes the
interpreter to exit the body of the switch statement. The case clauses in a switch
statement specify only the starting point of the desired code. The individual cases
are not independent blocks of code, and they do not have any implicit ending point.
Therefore, you must explicitly specify the end of each case with a break or related
statement. In the absence of break statements, a switch statement begins executing
code at the first statement after the matching case label and continues executing
statements until it reaches the end of the block. On rare occasions, it is useful to
write code like this that falls through from one case label to the next, but 99% of the
time you should be careful to end every case and default section with a statement
that causes the switch statement to stop executing. Normally you use a break state‐
ment, but return and throw also work.
a x
A switch statement can have more than one case clause labeling the same state‐
ment. Consider the switch statement in the following method:
boolean parseYesOrNoResponse ( char response ) {
switch ( response ) {
case 'y' :
case 'Y' : return true ;
case 'n' :
case 'N' : return false ;
default :
throw new IllegalArgumentException ( "Response must be Y or N" );
The switch statement and its case labels have some important restrictions. First,
the expression associated with a switch statement must have an appropriate type—
either byte , char , short , int (or their wrappers), or an enum type or a String . The
floating-point and boolean types are not supported, and neither is long , even
though long is an integer type. Second, the value associated with each case label
must be a constant value or a constant expression the compiler can evaluate. A case
label cannot contain a runtime expression involving variables or method calls, for
example. Third, the case label values must be within the range of the data type used
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