Transcript Interface & Abstract Class

Exception Handling
Background
The fact that software modules should be
robust enough to work under every
situation.
 The exception handling mechanism is key
to achieving this goal.

Exception Terminology

Using the exception handling mechanism
in Java involves:
◦ identifying exception conditions relevant to
the application;
◦ locating exception handlers to respond to
potential conditions; and
◦ monitoring when such conditions occur.
Exception Terminology
As with all representations in Java,
exception conditions are denoted by
objects.
 Similar with all objects, exceptions are
also defined by class constructs, but
inheriting attributes from the Exception
superclass.
 While exception objects may be identified
by object tags, additional attributes may
be included for custom manipulation.

Exception Terminology
Exception handling is dynamically enabled
for statement blocks within a try block.
 Within it, normal facilities and rules for
blocks apply but control-flow within may
be transferred to associated exception
handlers.
 An appropriate statement block prefixed
by a catch-clause is then executed when
the associated exception condition
occurs.

Exception Terminology

The occurrence of an exception
condition is indicated by a throwstatement. It allows for an exception
object to be dynamically propagated to
the most recent exception handler. Flowcontrol does not return following a
throw-statement. Instead, execution
control proceeds at the statement
following the try-block that handles the
exception.
Constructs and Exception
Semantics in Java

We now consider the language primitives
for realizing the exception handling
framework in Java. As seen,
◦ exception objects are defined via class constructs
that inherit from the Exception class;
◦ exception handling is enabled within a try-block,
with handlers indicated by catch clauses; and
◦ an exception condition is identified by a throw
statement. (Some predefined exception
conditions are thrown implicitly by the Java
Virtual Ma-chine.)
Defining Exception Objects

The smallest exception object in Java
merely extends from the Exception
superclass,as outlined in the class
definition for TransmissionError below:
Defining Exception Handlers

Exception handlers are introduced by the
catch-clause within a try-block prefix, of
which the following code fragment is
representative.
Raising Exceptions
An exception condition is ultimately
represented by an exception object
derived from the predefined Exception
class.
 A condition is made known by throwing
an appropriate object via a throw
statement, to be subsequently caught by
an associated handler.
