BLOCKS AND STATEMENTS

The

switch

 Statement

14.9

Every

case

 constant expression associated with a

switch

 statement must be

assignable ( 5.2) to the type of the

switch

Expression

.

No two of the

case

 constant expressions associated with a

switch

 statement

may have the same value.

At most one

default

 label may be associated with the same

switch

 state 

ment.

In C and C++ the body of a

switch

 statement can be a statement and state 

ments with

case

 labels do not have to be immediately contained by that state 

ment. Consider the simple loop:

for (i = 0; i < n; ++i) foo();

where

n

 is known to be positive. A trick known as

Duff's device

 can be used in C

or C++ to unroll the loop, but this is not valid Java code:

int q = (n+7)/8;

switch (n%8) {

case 0:

do {

foo();

//

Great C hack, Tom,

case 7:

foo();

//

but it's not valid in Java.

case 6:

foo();

case 5:

foo();

case 4:

foo();

case 3:

foo();

case 2:

foo();

case 1:

foo();

} while (  q >= 0);

}

Fortunately, this trick does not seem to be widely known or used. Moreover, it is

less needed nowadays; this sort of code transformation is properly in the province

of state of the art optimizing compilers.

When the

switch

 statement is executed, first the

Expression

 is evaluated. If

evaluation of the

Expression

 completes abruptly for some reason, the

switch

statement completes abruptly for the same reason. Otherwise, execution continues

by comparing the value of the

Expression

with each

case

 constant. Then there is a

choice:

If one of the

case

 constants is equal to the value of the expression, then we

say that the

case

 matches, and all statements after the matching

case

 label in

the switch block, if any, are executed in sequence. If all these statements com 

plete normally, or if there are no statements after the matching

case

 label,

then the entire

switch

 statement completes normally.

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