9.6. Design patterns

A software design pattern is a general, reusable solution to a common software problem. It is not an actual implementation, but rather an idea.

Design patterns gained popularity in computer science after the book Design Patterns: Elements of Reusable Object-Oriented Software was published in 1994 by (Gamma et al.). The book had 4 authors and together they became known as the “Gang of Four”, or even more briefly the “GoF”.

Design patterns are useful for several reasons:

… with patterns you get experience reuse.

—Head First Design Patterns, Freeman and Robson, 2014, p. 1.

9.6.1. Fixing an inheritance problem

Recall our flying penguin problem from the section on Inheritance:

classDiagram bird <|-- hawk bird <|-- owl bird <|-- penguin bird <|-- robin class bird { -wingspan double +fly() void +eat() int }

This type of design error is actually common.

It’s easy to overlook a design error, in this case flying penguins. Part of what makes these errors problematic is that design errors that actually get delivered in the final product are among the most expensive software errors to fix.

So how do we fix our bird design?

We could just override the fly method for penguins and have the override do nothing. This might solve our immediate problem, but it’s not a general solution to this problem:

  • It doesn’t work as well for non-void functions. In this case, fly – a void function – really could do nothing A non-void function must return something. If the function returns something, but that ‘something’ actually means ‘nothing’, then we have another design problem. This value now represents a marker that every caller of the function might need to handle and understand that no action was taken.

  • What happens when we need behaviors other than fly? If every time we have an exception case for a derived type and we create overrides that do nothing, then it can become difficult to understand the true capabilities of any object in the hierarchy just by looking that the interface.

    You have to examine the code to see how each (or if) an interface was implemented.

    Any design that requires us to look at the source code before we can completely understand it is not a design we want to use.

Another problem with the existing design is that it encourages duplication in derived classes. For example, let’s assume we need to model bird flying behavior. Most birds don’t just ‘fly’. They soar, flock, flit about, dive, or don’t fly at all. There are many thousands of different kinds of birds, but relatively few ways of flying.

We could decide to solve this problem using an interface:

classDiagram bird <|-- penguin bird <|-- hawk bird <|-- owl bird <|-- robin flyable <|.. hawk flyable <|.. owl flyable <|.. robin class bird { -wingspan double +eat() int } class hawk { +fly() void } class owl { +fly() void } class robin { +fly() void } class flyable { <<interface>> +fly() virtual void }

This does allow limiting the flying behavior to those birds that actually fly, but at a high cost. Now every bird that does fly needs to reimplement the code for fly(). Derived classes cannot inherit code from each other. Future maintenance of all the duplicated code could be expensive. Our current situation:

  • Not all birds fly, so inheritance is not the right choice

  • A simple interface solves the inheritance problem, but creates an unacceptable maintenance burden.

There must be a better option.

Note

Accommodating change

How can we isolate the parts of a system that change from the parts that do not change?

More to Explore

  • Software_design_pattern from Wikipedia.

  • Design Patterns Are Missing Language Features from the PortlandPatternRepository.

  • Revenge of the Nerds an excellent article written by Paul Graham in 2002. About the evolution of language and how modern languages are becoming more like Lisp – which was discovered in 1958. The end has a short criticism of patterns.

    Keep this date in mind when he uses phrases like ‘recently invented’: that’s still over 20 years ago!

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