Design Patterns 1

Software Developmentand Professional Practice

L8.5: Object-Oriented Design - Have I got a Strategy!

Intro to Development 2

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Design Patterns 3

SimUDuck!

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Now we want to fly!

So how to add flying to SimUDuck?

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Is this OK? Looks OK, right? So now we can add more ducks, right?

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What don’t these ducks do?

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So what to do?

Anything wrong with this?

So, inheritance might not be the right thing to use here Sound familiar?

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Back to OO Design principles...

take the parts of your system that vary and encapsulate them, so that later you can alter or extend the parts that vary without affecting those that don’t.

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So what changes?

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So what changes?

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So for the Ducks...

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Final version...

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And so what have we learned?

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Applicability of Strategy

Use the Strategy pattern when many related classes differ only in their behavior. you need different variants of an algorithm. an algorithm uses data that clients shouldn’t know

about. Use the Strategy pattern to avoid exposing complex, algorithm-specific data structures.

a class defines many behaviors, and these appear as multiple conditional statements in its operations. Instead of many conditionals, move related conditional branches into their own Strategy class.

(all this directly from GoF, page 316.)

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References Freeman, Eric and Freeman, Elisabeth, Head First Design

Patterns, O’Reilly, 2004. Dooley, John F., Software Development and Professional

Practice, Apress, 2011.