JAVA PROGRAMMING
The try Block
The first step in constructing an exception handler
is to enclose the statements that might throw an exception within a try
block. In general, a try block looks like this:
try {
Java statements
}
The segment of code labeled Java statements is composed of one or more
legal Java statements that could throw an exception.
The catch block
As you learned on the above, 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:
try {
. . .
} catch ( . . . ) {
. . .
} catch ( . . . ) {
. . .
} . . .
There can be no intervening code between the end of the try statement and
the beginning of the first catch statement. The general form of Java's
catch statement is:
catch (SomeThrowableObject variableName) {
Java statements
}
As you can see, the 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:
e.getMessage()
You access the instance variables and methods of exceptions in the same
manner that you access the instance variables and methods of other
objects. 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.
An example code using try and catch block
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