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Trail: Essential Classes
Lesson: Exceptions
Exceptions
What Is an Exception?
The Catch or Specify Requirement
Catching and Handling Exceptions
The try Block
The catch Blocks
The finally Block
Putting It All Together
Specifying the Exceptions Thrown by a Method
How to Throw Exceptions
Chained Exceptions
Creating Exception Classes
Unchecked Exceptions — The Controversy
Advantages of Exceptions
Summary
Questions and Exercises
Home Page > Essential Classes > Exceptions
Specifying the Exceptions Thrown by a Method

> The previous section showed how to write an exception handler for the `writeList` method in the `ListOfNumbers` class. Sometimes, it's appropriate for 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 the users of your package. In this case, it's better to not catch the exception and to allow a method further up the call stack to handle it.

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

> >     // Note: This method won't compile by design!
> >     public void writeList() {
> >         PrintWriter out =
> >                    new PrintWriter(new FileWriter("OutFile.txt"));
> >         for (int i = 0; i < SIZE; i++) {
> >             out.println("Value at: " + i + " = "
> >                          + vector.elementAt(i));
> >         }
> >         out.close();
> >     }

> To specify that `writeList` can throw two exceptions, add a `throws` clause to the method declaration for the `writeList` method. The `throws` clause comprises the `throws` keyword followed by a comma-separated list of all the exceptions thrown by that method. The clause goes after the method name and argument list and before the brace that defines the scope of the method; here's an example.

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

> Remember that `ArrayIndexOutOfBoundsException` is an unchecked exception; including it in the `throws` clause is not mandatory. You could just write the following.

> >     public void writeList() throws IOException {

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