Exception Handling: A Deeper Look - Java: How to Program Early Objects - page 450

we discuss the finally block in Section 11.6). Now the condition for the do … while loop
is false , and method main ends.
The try block and its corresponding catch and/or finally blocks form a try state-
ment . Do not confuse the terms “ try block” and “ try statement”—the latter includes the
try block as well as the following catch blocks and/or finally block.
As with any other block of code, when a try block terminates, local variables declared
in the block go out of scope and are no longer accessible; thus, the local variables of a try
block are not accessible in the corresponding catch blocks. When a catch block termi-
nates , local variables declared within the catch block (including the exception parameter
of that catch block) also go out of scope and are destroyed . Any remaining catch blocks in
the try statement are ignored , and execution resumes at the first line of code after the
try … catch sequence—this will be a finally block, if one is present.
Using the throws Clause
In method quotient (Fig. 11.3, lines 9-13), line 10 is known as a throws clause . It spec-
ifies the exceptions the method might throw if problems occur. This clause, which must
appear after the method's parameter list and before the body, contains a comma-separated
list of the exception types. Such exceptions may be thrown by statements in the method's
body or by methods called from there. We've added the throws clause to this application
to indicate that this method might throw an ArithmeticException . Method quotient 's
callers are thus informed that the method might throw an ArithmeticException . Some
exception types, such as ArithmeticException , are not required to be listed in the throws
clause. For those that are, the method can throw exceptions that have the is-a relationship
with the classes listed in the throws clause. You'll learn more about this in Section 11.5.
Error-Prevention Tip 11.1
Read the online API documentation for a method before using it in a program. The docu-
mentation specifies the exceptions thrown by the method (if any) and indicates reasons why
such exceptions may occur. Next, read the online API documentation for the specified excep-
tion classes. The documentation for an exception class typically contains potential reasons
that such exceptions occur. Finally, provide for handling those exceptions in your program.
When line 12 executes, if the denominator is zero, the JVM throws an Arithmetic-
Exception object. This object will be caught by the catch block at lines 42-47, which dis-
plays basic information about the exception by implicitly invoking the exception's
toString method, then asks the user to try again.
If the denominator is not zero, method quotient performs the division and returns
the result to the point of invocation of method quotient in the try block (line 29). Lines
30-31 display the result of the calculation and line 32 sets continueLoop to false . In this
case, the try block completes successfully, so the program skips the catch blocks and fails
the condition at line 48, and method main completes execution normally.
When quotient throws an ArithmeticException , quotient terminates and does not
return a value, and quotient 's local variables go out of scope (and are destroyed). If quo-
tient contained local variables that were references to objects and there were no other ref-
erences to those objects, the objects would be marked for garbage collection . Also, when an
exception occurs, the try block from which quotient was called terminates before lines
30-32 can execute. Here, too, if local variables were created in the try block prior to the
exception's being thrown, these variables would go out of scope.