648 Chapter 14 Exception Handling and Event Handling
14.4.1 Classes of Exceptions
All Java exceptions are objects of classes that are descendants of the Throw-
able class. The Java system includes two predefined exception classes that
are subclasses of Throwable: Error and Exception. The Error class and
its descendants are related to errors that are thrown by the Java run-time sys-
tem, such as running out of heap memory. These exceptions are never thrown
by user programs, and they should never be handled there. There are two
system-defined direct descendants of Exception: RuntimeException and
IOException. As its name indicates, IOException is thrown when an error
has occurred in an input or output operation, all of which are defined as meth-
ods in the various classes defined in the package java.io.
There are predefined classes that are descendants of RuntimeException.
In most cases, RuntimeException is thrown (by the JVM^4 ) when a user pro-
gram causes an error. For example, ArrayIndexOutOfBoundsException,
which is defined in java.util, is a commonly thrown exception that descends
from RuntimeException. Another commonly thrown exception that
descends from RuntimeException is NullPointer Exception.
User programs can define their own exception classes. The convention in
Java is that user-defined exceptions are subclasses of Exception.14.4.2 Exception Handlers
The exception handlers of Java have the same form as those of C++, except that
every catch must have a parameter and the class of the parameter must be a
descendant of the predefined class Throwable.
The syntax of the try construct in Java is exactly as that of C++, except for
the finally clause described in Section 14.4.6.14.4.3 Binding Exceptions to Handlers
Throwing an exception is quite simple. An instance of the exception class is
given as the operand of the throw statement. For example, suppose we define
an exception named MyException asclass MyException extends Exception {
public MyException() {}
public MyException(String message) {
super (message);
}
}This exception can be thrown with- The Java specification also requires JIT compilers to detect these exceptions and throw
RunTimeException when they occur.