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Trail: Essential Classes
Lesson: Exceptions
Section: Catching and Handling 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
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The catch Blocks

> You associate exception handlers with a `try` block by providing one or more `catch` blocks directly after the `try` block. No code can be between the end of the `try` block and the beginning of the first `catch` block.

> >     try {

> >     } catch (ExceptionType name) {

> >     } catch (ExceptionType name) {

> >     }

> Each `catch` block is an exception handler and handles the type of exception indicated by its argument. The argument type, `ExceptionType`, declares the type of exception that the handler can handle and must be the name of a class that inherits from the `Throwable` class. The handler can refer to the exception with `name`.

> The `catch` block contains code that is executed if and when the exception handler is invoked. The runtime system invokes the exception handler when the handler is the first one in the call stack whose `ExceptionType` matches the type of the exception thrown. The system considers it a match if the thrown object can legally be assigned to the exception handler's argument.

> The following are two exception handlers for the `writeList` method — one for two types of checked exceptions that can be thrown within the `try` statement.

> >     try {

> >     } catch (FileNotFoundException e) {
> >         System.err.println("FileNotFoundException: "
> >                             + e.getMessage());
> >         throw new SampleException(e);

> >     } catch (IOException e) {
> >         System.err.println("Caught IOException: "
> >                             + e.getMessage());
> >     }

> Both handlers print an error message. The second handler does nothing else. By catching any `IOException` that's not caught by the first handler, it allows the program to continue executing.

> The first handler, in addition to printing a message, throws a user-defined exception. (Throwing exceptions is covered in detail later in this chapter in the How to Throw Exceptions section.) In this example, when the `FileNotFoundException` is caught it causes a user-defined exception called `SampleException` to be thrown. You might want to do this if you want your program to handle an exception in this situation in a specific way.

> Exception handlers can do more than just print error messages or halt the program. They can do error recovery, prompt the user to make a decision, or propagate the error up to a higher-level handler using chained exceptions, as described in the Chained Exceptions section.

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