Exceptions and Exception Handling

To learn how to dispatch and respond to errors in our own programs, we'll return to the Box class example from Chapter 4. Recall that in the Box class, we defined a setWidth( ) method to set the width property of a Box instance. The setWidth( ) method checks whether a new width is within legal range before changing the width property of the Box instance. If the new width specified is not valid, no change is made, and the method signals a problem by returning the value false. The relevant Box class code follows. (Note that the Box class code in this example is incomplete; for now we're concentrating only on the portions of the class that deal with setting the width property.)

class Box {
private var width:Number;

public function setWidth (w:Number):Boolean {
if ((isNaN(w) || w == null) || (w <= 0 || w > Number.MAX_VALUE)) {
// Invalid data, so quit.
return false;
}
width = w;
return true;
}
}

When setWidth( ) returns the value false, it does so to indicate an error condition. Code that invokes the setWidth( ) method is expected to check setWidth( )'s return value and respond gracefully if there's a problem (i.e., if setWidth( ) returns false).

Let's now revise the setWidth( ) method so that it generates an exception, a standard form of error supported directly by ActionScript 2.0. Error dispatch and recovery with exceptions works in much the same way as the preceding setWidth( ) method; when a problem occurs, an error is signaled, and some error-recovery code is expected to handle it.

To generate an exception (i.e., signal an error) in our code, we use the throw statement, which takes the following form:

throw expression

where expression is a data value that describes the unusual or problematic situation. Using throw to signal an error is known as "throwing an exception." ActionScript allows any value to act as the expression of a throw statement. For example, the expression value could be the string literal "Something went wrong!" or it could be a numeric error code. However, the best practice is for expression to be an instance of the Error class (or an instance of a subclass of Error).

The Error class, introduced as a built-in class in Flash Player 7, is a standard class for representing error conditions. Instances of the Error class represent an error (a.k.a. an exception) in a program.