Flow Control - SCJP Sun Certified Programmer for Java Platform SE6 - page 222

Because the value of c on line 2 can only be RED , GREEN , or BLUE and the switch statement
has a case for all three of these outcomes, you can assert that line 12 is not reachable. This
example is typical of when to use an assertion. I insist with all certainty that line 12 will not
execute. Notice that if it does, an AssertionError is thrown because the boolean is false .
The only way this assertion would fail is if somehow the enum is modifi ed. Suppose you
are working on a project that uses the Colors enum, and during the development phase it is
discovered that yellow needs to be added to the list of colors. The assertion can help uncover
the ripple effect of such a change. Suppose the new version of Colors looks like this:
1. public enum Colors {
2. RED, GREEN, BLUE, YELLOW
3. }
See if you can determine the output of the following main method added to the
TestColors class:
public static void main(String [] args) {
Colors c = Colors.YELLOW;
testColor(c);
}
Because YELLOW is a new color and not one of the cases, the default block executes and
the assert fails. (It has to fail because it uses false for the boolean expression.) Assuming
assertions are enabled, an AssertionError is thrown and the following stack trace displays:
Exception in thread “main” java.lang.AssertionError: Invalid color
at TestColors.testColor(TestColors.java:12)
at TestColors.main(TestColors.java:18)
A control fl ow assertion is a common use of assert statements. When possible, place an
assert statement at any location in your code that you assume will not be reached.
Assertions Should Not Alter Outcomes
Because assertions can and probably will be turned off in a production environment, your
assertions should not contain any business logic that affects the outcome of your code.
For example, the following assertion is not a good design because it alters the value of a
variable:
int x = 10;
assert ++x > 10; //Not a good design!
When assertions are turned on, x is incremented to 11 , but when assertions are turned
off, the value of x is 10 . Therefore, the outcome of the code will be different, and assert
statements should have no effect on your application if they are turned off, so this is not
a good use of assertions.