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PATTERNS04 - Structural Design Patterns

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An introduction to structural design patterns in object orientation. Suitable for intermediate to advanced computing students and those studying software engineering.

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PATTERNS04 - Structural Design Patterns

  1. 1. Structural Design Patterns Michael Heron
  2. 2. Introduction  Structural design patterns emphasize relationships between entities.  These can be classes or objects.  They are designed to help deal with the combinatorial complexity of large object oriented programs.  These often exhibit behaviours that are not immediately intuitive.
  3. 3. Structural Patterns  Common to the whole philosophy behind structural patterns is the separation between abstraction and implementation.  This is a good guideline for extensible design.  Structural patterns subdivide into two broad subcategories.  Class structural patterns  Where the emphasis is on the relationship between classes  Object structural patterns  The emphasis is on consistency of interaction and realization of new functionality.
  4. 4. Façade  When a model is especially complex, it can be useful to add in an additional pattern to help manage the external interface of that model.  That pattern is called a façade.  A façade sits between the view/controller and provides a stripped down or simplified interface to complex functionality.  There are costs to this though in terms of coupling and cohesion.  A façade is a structural pattern.
  5. 5. Façade  A façade provides several benefits  Makes software libraries easier to use by providing helper methods  Makes code more readable  Can abstract away from the implementation details of a complex library or collection of classes.  Can work as a wrapper for poorly designed APIs, or for complex compound relationships between objects.
  6. 6. Façade Example public class FacadeExample { SomeClass one; SomeOtherClass two; SomeKindOfConfigClass three; public SomeOtherClass handleInput (String configInfo) { three = SomeKindOfConfigClass (configInfo) one = new SomeClass (configInfo); two = one.getSomethingOut (); return two; } }
  7. 7. Façade Example public class FacadeExample { public SomeOtherClass handleInput (String configInfo) { return myFacade.doSomeMagic (configInfo); } } public class Facade { SomeClass one; SomeOtherClass two; SomeKindOfConfigClass three; public SomeOtherClass doSomeMagic (String configInfo) { three = SomeKindOfConfigClass (configInfo) one = new SomeClass (configInfo); two = one.getSomethingOut (); return two; } }
  8. 8. Façade  The more code that goes through the façade, the more powerful it becomes.  If just used in one place, it has limited benefit.  Multiple objects can make use of the façade.  Greatly increasing the easy of development and reducing the impact of change.  All the user has to know is what needs to go in, and what comes out.  The façade hides the rest
  9. 9. Downsides  This comes with a necessary loss of control.  You don’t really know what’s happening internally.  Facades are by definition simplified interfaces.  So you may not be able to do Clever Stuff when blocked by one.  Facades increase structural complexity.  It’s a class that didn’t exist before.  Facades increase coupling and reduce cohesion.  They often have to link everywhere, and the set of methods they expose often lack consistency
  10. 10. The Adapter  The Adapter design pattern is used to provide compatibility between incompatible programming interfaces.  This can be used to provide legacy support, or consistency between different APIs.  These are also sometimes called wrappers.  We have a class that wraps around another class and presents an external interface.
  11. 11. The Adapter  Internally, an adapter can be as simple as a composite object and a method that handles translations.  We can combine this with other design patterns to get more flexible solutions.  For example, a factory for adapters  Or adapters that work using the strategy pattern.  It is the combination of design patterns that has the greatest potential in design.
  12. 12. Simple Example abstract class Shape { abstract void drawShape (Graphics g, int x1, int x2, int y1, int y2); } public class Adapter { private Shape sh; public void drawShape (int x, int y, int len, int ht, Graphics g) { sh.drawShape (g, x, x+ht, y, y+len); } }
  13. 13. Adapters and Facades  What’s the difference between a façade and an adapter?  A façade presents a new simplified API to external objects.  An adapter converts an existing API to a common standard.  The Façade creates the programming interface for the specific combination of objects.  The adapter simply enforces consistency between incompatible interfaces.
  14. 14. The Flyweight  Object oriented programming languages provide fine-grained control over data and behaviours.  But that flexibility comes at a cost.  The Flyweight pattern is used to reduce the memory and instantiation cost when dealing with large numbers of finely- grained objects.  It does this by sharing state whenever possible.
  15. 15. Scenario  Imagine a word processor.  They’re pretty flexible. You can store decoration detail on any character in the text.  How is this done?  You could represent each character as an object.  You could have each character contain its own font object…  … but that’s quite a memory overhead.  It would be much better if instead of holding a large font object, we held only a reference to a font object.
  16. 16. The Flyweight  The Flyweight pattern comes in to reduce the state requirements here.  It maintains a cache of previously utilised configurations or styles.  Each character is given a reference to a configuration object.  When a configuration is applied, we check the cache to see if it exists.  If it doesn’t, it creates one and add it to the cache.  The Flyweight dramatically reduces the object footprint.  We have thousands of small objects rather than thousands of large objects.
  17. 17. Before and After public class MyCharacter { char letter; Font myFont; void applyDecoration (string font, int size); myFont = new Font (font, size); } } public class MyCharacter { char letter; Font myFont; void applyDecoration (string font, int size); myFont = FlyweightCache.getFont (font, size); } }
  18. 18. Implementing a Flyweight  The flyweight patterns makes no implementation assumptions.  A reasonably good way to do it is through a hash map or other collection.  Standard memoization techniques can be used here.  When a request is made, check the cache.  If it’s there, return it.  If it’s not, create it and put it in the cache and return the new instance.
  19. 19. Limitations of the Flyweight Pattern  Flyweight is only an appropriate design pattern when object references have no context.  As in, it doesn’t matter to what they are being applied.  A font object is a good example.  It doesn’t matter if it’s being applied to a number, a character, or a special symbol.  A customer object is a bad example.  Each customer is unique.
  20. 20. The Composite Pattern  We often have to manipulate collections of objects when programming.  The composite pattern is designed to simplify this.  Internally, it represents data as a simple list or other collection.  Requires the use of polymorphism to assure structural compatability.  Externally it presents an API to add and remove objects.  And also to execute operations on the collection as a whole.
  21. 21. The Composite Pattern public class ShapeCollection implements Shape() { ArrayList shapes = new ArrayList(); void addShape (Shape s) { shapes.Add (s); } void removeShape (Shape s) { shapes.Remove (s); } void draw() { foreach (Shape s in shapes) { s.draw(); } } void setColour (Colour c) { foreach (Shape s in shapes) { s.setColour (c); } } }
  22. 22. The Composite Pattern public MainProgram() { Circle circle = new Circle(); Rectangle rect = new Rectangle(); Triangle tri = new Triangle(); ShapeCollection myCollection = new ShapeCollection(); ShapeCollection overallScene = new ShapeCollection(); myCollection.addShape (circle); myCollection.addShape (rect); overallScene.addShape (myCollection); overallScene.addShape (tri); myCollection.setColour (Colour.RED); overallScene.draw(); }
  23. 23. Why Use Composite?  Sometimes we need to be able to perform operations on groups of objects as a whole.  We may wish to move a group of shapes in a graphics package as one example.  These often exist side by side with more primitive objects that get manipulated individually.  Having handling code for each of these conditions is bad design.
  24. 24. The Composite  The composite allows us to treat collections and individual objects through one consistent interface.  We don’t need to worry about which we are dealing with at any one time.  It works by ensuring that the collection implements the common interface shared by all its constituent bits.  The relationship is recursive if done correctly.
  25. 25. Summary  Structural patterns are the last of the families of design patterns we are going to look at.  We use an adapter to deal with incompatible APIs.  We use a bridge to decouple abstraction from implementation.  Implementation is very similar to strategy, only the intent is unique.  Flyweight patterns are used to reduce processing and memory overheads.  Composites are used to allow recursive and flexible aggregate manipulation of objects.

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