Multi-format calculator - Software Development Case Studies in Java - page 152

well-known Strategy pattern (Gamma et al . 1995). In addition, we have seen
how exceptions can help in dealing with abnormal situations and how they
allow a more readable and clean handling of errors.

Idiom Class generalization

Context Many classes have common features.

Problem The replication of the same features makes classes less

understandable and maintainable.

Forces or tradeoffs The same attribute or method is repeated many times in

different classes.
Some operations are common to many classes.

Solution Create a superclass of all the existing similar classes.

Migrate the common characteristics into the superclass.

Force resolution Common features are not replicated. The superclass

becomes the single point of change. Derived classes are
simpler. Code reuse is enforced. Gathering common
features makes the structure more comprehensible.

Design rationale We find commonalities among existing classes and make

them into a single class.

Pattern Strategy

Context A class embeds a behaviour that can be chosen and or

changed at run time.

Problem Switching the behaviour dynamically.

Lets the behaviour vary independently from clients that
use it.

Forces or tradeoffs The clients need a stable interface. The behaviour must be

changeable.
The multiple behaviours must be evident from the
structure of the classes.

Solution Define a common interface for all the different behaviours

( Strategy ). For each behaviour define a class that
implements the interface ( ConcreteStrategyA ,
ConcreteStrategyB , ...). The behaviours are used by a
client that defines the context within which they operate
( Context ). To select a behaviour an object of one of the
behaviour classes can be linked to the Context .

! strategy

<<interface>>
Strategy

Context

! operation()

ConcreteStrategyA ConcreteStrategyB

! operation() ! operation()