The
The
try
statement defines the scope of its associated exception handlers. You associate exception handlers with a try
statement by providing one or more catch
blocks directly after the try
block:There can be no intervening code between the end of thetry { . . . } catch ( . . . ) { . . . } catch ( . . . ) { . . . } . . .
try
statement and the beginning of the first catch
statement. The general form of Java's catch
statement is:As you can see, thecatch (SomeThrowableObject variableName) { Java statements }
catch
statement requires a single formal argument. The argument to the catch
statement looks like an argument declaration for a method. The argument type, SomeThrowableObject, declares the type of exception that the handler can handle and must be the name of a class that inherits from the Throwable
class defined in the java.lang
package. When Java programs throw an exception they are really just throwing an object, and only objects that derive from Throwable
can be thrown. You'll learn more about throwing exceptions in How to Throw Exceptions.variableName is the name by which the handler can refer to the exception caught by the handler. For example, the exception handlers for the writeList
method (shown later) each call the exception's getMessage
method using the exception's declared name e
:You access the instance variables and methods of exceptions in the same manner that you access the instance variables and methods of other objects.e.getMessage()
getMessage
is a method provided by the Throwable
class that prints additional information about the error that occurred. The Throwable
class also implements two methods for filling in and printing the contents of the execution stack when the exception occurred. Subclasses of Throwable
can add other methods or instance variables. To find out what methods an exception implements, check its class definition and definitions for any of its ancestor classes.The catch
block contains a series of legal Java statements. These statements are 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 type matches that of the exception thrown.The
writeList
method from the ListOfNumbers class uses two exception handlers for its try
statement, with one handler for each of the two types of exceptions that can be thrown within the try
block -- ArrayIndexOutOfBoundsException
and IOException
.try { . . . } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught ArrayIndexOutOfBoundsException: " + e.getMessage()); } catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage()); }
Catching Multiple Exception Types with One Handler
The two exception handlers used by thewriteList
method are very specialized. Each handles only one type of exception. The Java language allows you to write general exception handlers that handle multiple types of exceptions.As you know, Java exceptions areThrowable
objects; they are instances ofThrowable
or a subclass ofThrowable
. The Java packages contain numerous classes that derive fromThrowable
and thus, build a hierarchy ofThrowable
classes.
Your exception handler can be written to handle any class that inherits fromThrowable
. If you write a handler for a "leaf" class (a class with no subclasses), you've written a specialized handler: it will only handle exceptions of that specific type. If you write a handler for a "node" class (a class with subclasses), you've written a general handler: it will handle any exception whose type is the node class or any of its subclasses.Let's modify thewriteList
method once again. Only this time, let's write it so that it handles bothIOExceptions
andArrayIndexOutOfBoundsExceptions
. The closest common ancester ofIOException
andArrayIndexOutOfBoundsException
is theException
class. An exception handler that handles both types of exceptions looks like this:
Thetry { . . . } catch (Exception e) { System.err.println("Exception caught: " + e.getMessage()); }class is pretty high in the Throwable
class hierarchy. So in addition to theIOException
andArrayIndexOutOfBoundsException
types that this exception handler is intended to catch, it will catch numerous other types. Generally speaking, your exception handlers should be more specialized. Handlers that can catch most or all exceptions are typically useless for error recovery because the handler has to determine what type of exception occurred anyway to determine the best recovery strategy. Also, exception handlers that are too general can make code more error prone by catching and handling exceptions that weren't anticipated by the programmer and for which the handler was not intended.
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