11

C H A P T E R

Exceptions

If anything can go wrong, it will.

 Finagle's Law

(often incorrectly attributed to Murphy, whose law is rather

different which only goes to show that Finagle was right)

W

HEN a Java program violates the semantic constraints of the Java language, a

Java Virtual Machine signals this error to the program as an

exception

. An exam 

ple of such a violation is an attempt to index outside the bounds of an array. Some

programming languages and their implementations react to such errors by

peremptorily terminating the program; other programming languages allow an

implementation to react in an arbitrary or unpredictable way. Neither of these

approaches is compatible with the design goals of Java: to provide portability and

robustness. Instead, Java specifies that an exception will be thrown when semantic

constraints are violated and will cause a non local transfer of control from the

point where the exception occurred to a point that can be specified by the pro 

grammer. An exception is said to be

thrown

 from the point where it occurred and

is said to be

caught

 at the point to which control is transferred.

Java programs can also throw exceptions explicitly, using

throw

 statements

( 14.16). This provides an alternative to the old fashioned style of handling error

conditions by returning funny values, such as the integer value

 1

 where a nega 

tive value would not normally be expected. Experience shows that too often such

funny values are ignored or not checked for by callers, leading to programs that

are not robust, exhibit undesirable behavior, or both.

Every exception is represented by an instance of the class

Throwable

 or one

of its subclasses; such an object can be used to carry information from the point at

which an exception occurs to the handler that catches it. Handlers are established

by

catch

 clauses of

try

 statements ( 14.18). During the process of throwing an

exception, a Java Virtual Machine abruptly completes, one by one, any expres 

sions, statements, method and constructor invocations, static initializers, and field

initialization expressions that have begun but not completed execution in the cur 

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