public class CompositeSpec in Java

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public class CompositeSpec
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extends Spec
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Now the compiler is happy, as is the test code 5 Because the List-based selectBy( ) method is now only one line of code that calls the one- pec selectBy( ) method, I inline it S by applying Inline Method [F] Client code that used to call theList-based selectBy( ) now calls the one-Spec selectBy( ) method Here's an example of such a change:
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public class ProductRepositoryTest public void testFindByColorSizeAndBelowPrice() { List specs = new ArrayList(); specsadd(new ColorSpec(Colorred)); specsadd(new SizeSpec(ProductSizeSMALL)); specsadd(new BelowPriceSpec(1000));
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List foundProducts = repositoryselectBy(specs);
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List foundProducts = repositoryselectBy(
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new CompositeSpec(specs));
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There's now only one selectBy( ) method that accepts Spec objects like ColorSpec, SizeSpec, or the new CompositeSpec This is a useful start However, to build Composite structures that support product searches like productgetColor() != targetColor || productgetPrice() < targetPrice, there is a need for classes likeNotSpec and OrSpec I won't show how they're created here; you can read about them in the refactoring Replace Implicit Language with Interpreter (269) 6 The final step involves applying Encapsulate Collection [F] on the collection inside of CompositeSpec I do this to make CompositeSpec more type-safe (ie, to prevent clients from adding objects to it that aren't a subclass of Spec) I begin by defining theadd(Spec spec) method:
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public class CompositeSpec extends Spec private List specs;
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public void add(Spec spec) { specsadd(spec); }
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Next, I initialize specs to an empty list:
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public class CompositeSpec extends Spec private List specs
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= new ArrayList();
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Now comes the fun part I find all callers ofCompositeSpec's constructor and update them to call a new, default CompositeSpec constructor as well as the new add( ) method Here is one such caller and the updates I make to it:
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public class ProductRepositoryTest public void testFindByColorSizeAndBelowPrice()
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List specs = new ArrayList(); CompositeSpec specs = new CompositeSpec();
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specsadd(new ColorSpec(Colorred)); specsadd(new SizeSpec(ProductSizeSMALL)); specsadd(new BelowPriceSpec(1000)); List foundProducts = repositoryselectBy(
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specs);
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I compile and test to confirm that the changes work Once I've updated all other clients, there are no more callers to CompositeSpec's constructor that take aList So I delete it:
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public class CompositeSpec extends Spec
public CompositeSpec(List specs) {
thisspecs = specs;
Now I update CompositeSpec's getSpecs( ) method to return an unmodifiable version of specs:
public class CompositeSpec extends Spec
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private List specs = new ArrayList(); public List getSpecs() return
CollectionsunmodifiableList(specs);
I compile and test to confirm that my implementation ofEncapsulate Collection works It doesCompositeSpec is now a fine implementation of the Composite pattern:
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Replace Hard-Coded Notifications with Observer
Subclasses are hard-coded to notify a single instance of another class
Remove the subclasses by making their superclass capable of notifying one or more instances of any class that implements an Observer interface
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Motivation
Knowing when to refactor to an Observer [DP] first involves understanding when you don't need an Observer Consider the case of a single instance of a class called Receiver that changes when an instance of a class calledNotifier changes, as shown in the following diagram
In this case, the Notifier instance holds onto aReceiver reference and is hard-coded to notify that reference when it receives new information Such a tight coupling between Notifier and Receiver makes sense when oneNotifier instance must notify only one Receiver instance If that circumstance changes and a Notifier instance must notify numerous Receiver instances, or instances of other classes, the design must evolve This is exactly what occurred on Kent Beck and Erich Gamma's JUnit framework [Beck and Gamma] When users of the framework needed more than one party to observe changes to a TestResult instance, a hard-coded notification was refactored to use the Observer pattern (see the Example section for details) Every implementation of the Observer pattern leads to loose coupling between a subject (a class that is the source of notifications) and its observers The Observer interface makes this loose coupling possible To be notified of new information, a class need only implement the Observer interface and register itself with a subject The subject, in turn, holds onto a collection of instances that implement the Observer interface, notifying them when changes occur Classes that play the role of subject must contain a method for adding Observers and can optionally contain a method for removing Observers If there is never a need to remove Observers during the life of a subject instance, there's no need to implement the remove method Although this seems like basic common sense, many programmers fall into the trap of implementing this pattern exactly as they see its structure defined on class diagrams in books Two common Observer implementation problems to watch out for involve cascading notifications and memory leaks Cascading notifications occur when a subject notifies an observer, which, by also playing the role of subject, notifies other observers, and so on The result is an overly complicated design that's difficult to debug A Mediator [DP] implementation may help improve such code Memory leaks occur when an observer instance is not garbage collected because the instance is still referenced by a subject If you remember to always remove your observers from their subjects, you'll avoid memory leaks The Observer pattern is used often Because it isn't difficult to implement, you may be tempted to use this pattern before it's actually needed Resist that temptation! If you begin with a hard-coded notification, you can always evolve a design to use an Observer when you genuinely need one
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