Software Architecture Design Patterns in Java

37 NULL OBJECT

This pattern was previously described in Woolf96, Grand98.

DESCRIPTION

The term null is used in most computer programming languages to refer to a nonexisting object. The Null Object pattern is applicable when a client expects to use different subclasses of a class hierarchy to execute different behavior and refers these subclasses as objects of the parent class type. At times, it may be possible that a subclass instance may not be available when the client expects one. In such cases, what a client object receives is a nonexisting object or null. When a null is returned, the client cannot invoke methods as it would if a real object is returned. Hence the client needs to check to make sure that the object is not null before invoking ...