Flow of Control - Java - page 102

When a switch statement is executed, one of a number of different branches is exe-
cuted. The choice of which branch to execute is determined by a controlling expres-
sion given in parentheses after the keyword switch . Following this are a number of
occurrences of the reserved word case followed by a constant and a colon. These
constants are called case labels . The controlling expression for a switch statement
must be one of the types char , int , short , or byte . 1 The case labels must all be of the
same type as the controlling expression. No case label can occur more than once,
because that would be an ambiguous instruction. There may also be a section labeled
default: , which is usually last.
When the switch statement is executed, the controlling expression is evaluated and
the computer looks at the case labels. If it finds a case label that equals the value of the
controlling expression, it executes the code for that case label.
The switch statement ends when either a break statement is executed or the end of the
switch statement is reached. A break statement consists of the keyword break followed
by a semicolon. When the computer executes the statements after a case label, it continues
until it reaches a break statement. When the computer encounters a break statement, the
switch statement ends. If you omit the break statements, then after executing the code for
one case, the computer will go on to execute the code for the next case.
Note that you can have two case labels for the same section of code, as in the fol-
lowing portion of a switch statement:

controlling
expression

case labels

break
statement

case 'A':
case 'a':
System.out.println("Excellent. You need not take the final.");
break ;

Because the first case has no break statement (in fact, no statements at all), the effect is
the same as having two labels for one case, but Java syntax requires one keyword case
for each label, such as 'A' and 'a' .
If no case label has a constant that matches the value of the controlling expression,
then the statements following the default label are executed. You need not have a
default section. If there is no default section and no match is found for the value of
the controlling expression, then nothing happens when the switch statement is exe-
cuted. However, it is safest to always have a default section. If you think your case
labels list all possible outcomes, you can put an error message in the default section.
The default case need not be the last case in a switch statement, but making it the
last case, as we have always done, makes the code clearer.
A sample switch statement is shown in Display 3.2. Notice that the case labels do
not need to be listed in order and need not span a complete interval.

default

1 As we will see in Chapter 6, the type may also be an enumerated type, but we do not cover enumer-
ated types until Chapter 6.