Control Flow, Looping, and Branching - Java 7 for Absolute Beginners - page 78

Note the default case at the end. That's a switch block's way of letting you do something with input
that doesn't match any of the cases. In this example, we use it to catch the values between 18 and 24
(inclusive). We can do that because we know we won't get any other values. If we aren't sure that we
wouldn't get an unexpected value, we would probably throw an exception in the default case, to let
whatever code uses the switch block know that we found a problem.
One issue with a switch statement is that it cannot group its possible values (other than through the
default keyword). Consequently, we end up with a lot of case labels that appear to do nothing. In fact,
each group of case statements shares the next block of code they encounter within the switch statement.
In this case, that means values 1 through 11 share the code that adds “morning” to the greeting and
values 12 through 17 share the code that adds “afternoon” to the greeting.
Another issue with switch statements is that each block of code generally requires a break
statement. Each break statement makes the program jump from the break statement to the end of the
switch statement. Consider what happens if we assume the input is 10. As written, the output is “Good
morning.” Without the break statements, the output is “Good morningafternoonevening.” That is, each
of the blocks is run. This happens because any case label without a break statement falls through to the
next case statement. That's the mechanism that lets multiple case labels share one block of code. With
no break statements, every block of code in the switch statement gets run. Although running more than
one block of code might be useful in some cases, it's generally an error (and can be hard to find because
the code probably looks right).
A final issue is the default keyword. A switch statement doesn't require a default section. However,
switch statements often do include default sections. A default section performs the same function as a
final else statement in a group of if - else statements. As with the final else , you can use a default
section for any remaining values, provided you are sure you won't get an unexpected value for the
switch statement's argument. When you can't be sure of your input, use the default block to trap
unexpected values and take some appropriate action (often either doing nothing or throwing an
exception). default sections don't require break statements (the code “falls through” to the end of the
switch statement anyway), but it's customary (and it's custom because it's a good idea) to add one, for
consistency and readability.
Let's consider another example in Listing 5-6 of a switch statement, this one being parallel in
function to Listing 5-4.
Listing 5-6. Using a switch statement without a default section
GregorianCalendar gregorianCalendar = new GregorianCalendar();
int day = gregorianCalendar.get(Calendar.DAY_OF_WEEK);
String greeting = "Good ";
switch (day) {
case 1: {
greeting += "Sunday";
}
case 2: {
greeting += "Monday";
}
case 3: {
greeting += "Tuesday";
}
case 4: {
greeting += "Wednesday";
}