11.1

The Causes of Exceptions

EXCEPTIONS

rent thread. This process continues until a handler is found that indicates that it

handles that particular exception by naming the class of the exception or a super 

class of the class of the exception. If no such handler is found, then the method

uncaughtException

 ( 20.21.31) is invoked for the

ThreadGroup

 that is the par 

ent of the current thread thus every effort is made to avoid letting an exception

go unhandled.

The Java exception mechanism is integrated with the Java synchronization

model ( 17), so that locks are released as

synchronized

 statements ( 14.17) and

invocations of

synchronized

 methods ( 8.4.3.5,  15.11) complete abruptly.

This chapter describes the different causes of exceptions ( 11.1). It details

how exceptions are checked at compile time ( 11.2) and processed at run time

( 11.3). A detailed example ( 11.4) is then followed by an explanation of the

exception hierarchy and the standard exception classes ( 11.5).

11.1   The Causes of Exceptions

If we do not succeed, then we run the risk of failure.

 J. Danforth Quayle (1990)

An exception is thrown for one of three

reasons

:

An abnormal execution condition was synchronously detected by a Java Vir 

tual Machine. Such conditions arise because:

N

evaluation of an expression violates the normal semantics of the Java lan 

guage, such as an integer divide by zero, as summarized in  15.5

N

an error occurs in loading or linking part of the Java program ( 12.2,  12.3)

N

some limitation a resource is exceeded, such as using too much memory

These exceptions are not thrown at an arbitrary point in the program, but

rather at a point where they are specified as a possible result of an expression

evaluation or statement execution.

A

throw

 statement ( 14.16) was executed in Java code.

An asynchronous exception occurred either because:

N

the method

stop

 of class

Thread

 ( 20.20.16) was invoked

N

an internal error has occurred in the virtual machine ( 11.5.2.2)

Exceptions are represented by instances of the class

Throwable

 and instances

of its subclasses. These classes are, collectively, the

exception classes

.

202