The Java Language Specification, Third Edition

14.11 The switch Statement BLOCKS AND STATEMENTS
378
• 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 statement.
DISCUSSION
The prohibition against using null as a switch label prevents one from writing code that can
never be executed. If the switch expression is of a reference type, such as a boxed primitive
type or an enum, a run-time error will occur if the expression evaluates to null at run-time.
It follows that if the switch expression is of an enum type, the possible values of the
switch labels must all be enum constants of that type.
Compilers are encouraged (but not required) to provide a warning if a switch on an
enum-valued expression lacks a default case and lacks cases for one or more of the enum
type’s constants. (Such a statement will silently do nothing if the expression evaluates to
one of the missing constants.)
In C and C++ the body of a switch statement can be a statement and statements
with case labels do not have to be immediately contained by that statement.
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 code in the Java programming language:
int q = (n+7)/8;
switch (n%8) {
case 0: do {foo(); // Great C hack, Tom,
case 7: foo(); // but it’s not valid here.
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.
DRAFT