Flow Control - SCJP Sun Certified Programmer for Java Platform SE6 - page 224

Validating Method Parameters
Do not use assertions to check for valid arguments passed in to a method. Use an
IllegalArgumentException instead. For example, the constructor of Rectangle should
throw an IlllegalArgumentException when either the width or height is negative:
public Rectangle(int width, int height) {
if(width < 0 || height < 0) {
throw new IllegalArgumentException();
}
this.width = width;
this.height = height;
}
This constructor greatly improves the reliability of the Rectangle class because there
is no way to change the fi eld's width and height except in the constructor. Remember,
assertions are for situations where you are certain of something and you just want to
verify it. You cannot be certain that someone instantiating a Rectangle will pass in
positive values. However, with the Rectangle constructor defi ned here, I should be able
to assert with a great deal of certainty that invoking isValid on any Rectangle object
will return true .
Assertions are used for debugging purposes, allowing you to verify that something you
think is true during the coding phase is actually true at runtime. The next section covers
exceptions, which affect the fl ow of control of your application similar to failed assertions.
Unlike assertions, exceptions are situations that arise at runtime that cannot be predicted
during the coding phase.
Overview of Exceptions
This section addresses the exam objectives that state you should be able to “develop code
that makes use of exception handling clauses (try, catch, fi nally), and declares methods and
overriding methods that throw exceptions,” as well as “recognize the effect of an exception
arising at a specifi ed point in a code fragment.” An exception is an event that occurs during
the execution of a program that disrupts the normal fl ow of control. In Java, an exception
is an object that a method “throws” down the method call stack by handing it to the JVM
and letting the JVM search for a handler. As the exception object travels down the methods
on the call stack, any method along the way has the opportunity to catch the exception.
Once caught, the method can obtain information about the problem and attempt to fi x it,
log the error in a fi le, or simply ignore the exception altogether. A caught exception can
also be rethrown, or a method can throw a different type of exception.