The JavaTM Tutorial

| Start of Tutorial > Start of Trail > Start of Lesson | Search

Trail: Essential Java Classes
Lesson: Handling Errors with Exceptions

## Specifying the Exceptions Thrown by a Method

> [PENDING: this leaves the file descriptor open. Need a new example to show this.]

> The previous section showed you how to write an exception handler for the `writeList` method in the `ListOfNumbers` class. Sometimes, it's appropriate for your code to catch exceptions that can occur within it. In other cases, however, it's better to let a method further up the call stack handle the exception. For example, if you were providing the `ListOfNumbers` class as part of a package of classes, you probably couldn't anticipate the needs of all of the users of your package. In this case, it's better to not catch the exception and to allow someone further up the call stack to handle it.

> If the `writeList` method doesn't catch the exceptions that can occur within it, then the `writeList` method must specify that it can throw them. Let's modify the `writeList` method to specify the exceptions that it can throw. To remind you, here's the original version of the `writeList` method:

> >     public void writeList() {
> >         PrintWriter out = new PrintWriter(
> >                               new FileWriter("OutFile.txt"));

> >         for (int i = 0; i < size; i++)
> >             out.println("Value at: " + i + " = " + victor.elementAt(i));
> >     }

> As you recall, the `new FileWriter("OutFile.txt")` statement might throw an `IOException` (which is not a runtime exception). The `victor.elementAt(i)` statement can throw an `ArrayIndexOutofBoundsException` (which, as a subclass of `RuntimeException`, is a runtime exception).

> To specify that `writeList` throws these two exceptions, you add a `throws` clause to the method signature for the `writeList` method. The `throws` clause is composed of the `throws` keyword followed by a comma-separated list of all the exceptions thrown by that method. The `throws` clause goes after the method name and argument list and before the curly bracket that defines the scope of the method. Here's an example:

> >     public void writeList throws IOException,
> >                                  ArrayIndexOutOfBoundsException {

> Remember that `ArrayIndexOutofBoundsException` is a runtime exception, so you don't have to specify it in the `throws` clause, although you can.

| Start of Tutorial > Start of Trail > Start of Lesson | Search