parentNode = currentNode; currentNode = childNode; historypush(currentNode); } // etc in Java

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parentNode = currentNode; currentNode = childNode; historypush(currentNode); } // etc
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The addValue( ) method is more tricky to move to ElementAdapter because it relies on a field withinElementAdapter called document:
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public class DOMBuilder extends AbstractBuilder
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private Document document;
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public void addValue(ElementAdapter current, String value) { currentgetElement()appendChild(
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documentcreateTextNode(value));
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I don't want to pass a field of type Document to the addValue( ) method on ElementAdapter because if I do so, that method will move further away from the target, which is the addValue( ) method on XMLNode:
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public interface XMLNode public abstract void addValue(String value);
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At this point I decide to pass an instance ofDocument to ElementAdapter via its constructor:
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public class ElementAdapter Element element;
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Document document;
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public ElementAdapter(Element element,
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Document document) {
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thiselement = element;
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thisdocument = document;
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And I make the necessary changes inDOMBuilder to call this updated constructor Now I can easily moveaddValue( ):
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public class ElementAdapter
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public void addValue(String value) { getElement()appendChild(documentcreateTextNode(value)); }
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6 Now I make ElementAdapter implement the XMLNode interface This step is straightforward, except for a small change to the add( ) method to allow it to call the getElement() method, which is not part of the XMLNode interface:
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public class ElementAdapter
implements XMLNode
public void add(XMLNode child) {
ElementAdapter childElement = (ElementAdapter)child;
getElement()appendChild(
childElementgetElement());
7 The final step is to update DOMBuilder so that all of its ElementAdapter fields, local variables, and parameters change their type to XMLNode:
public class DOMBuilder extends AbstractBuilder private Document document; private
XMLNode rootNode;
private
XMLNode parentNode;
private
XMLNode currentNode;
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public void addChild(String childTagName) {
XMLNode childNode =
new ElementAdapter(documentcreateElement(childTagName), document); } protected void init(String rootName) { document = new DocumentImpl(); rootNode = new ElementAdapter(documentcreateElement(rootName), document); documentappendChild(
((ElementAdapter)rootNode)getElement());
}
At this point, by adaptingElement in DOMBuilder, the code in XMLBuilder is so similar to that of DOMBuilder that it makes sense to pull up the similar code to AbstractBuilder I achieve that by applyingForm Template Method (205) and Introduce Polymorphic Creation with Factory Method (88) The following diagram shows the result
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Extract Adapter
One class adapts multiple versions of a component, library, API, or other entity
Extract an Adapter for a single version of the component, library, API, or other entity
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Motivation
While software must often support multiple versions of a component, library, or API, code that handles these versions doesn't have to be a confusing mess Yet I routinely encounter code that attempts to handle multiple versions of something by overloading classes with version-specific state variables, constructors, and methods Accompanying such code are comments like "This is for version X please delete this code when we move to version Y!" Sure, like that's ever going to happen Most programmers won't delete the version X code for fear that something they don't know about still relies on it So the comments don't get deleted, and many versions supported by the code
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remain in the code Now consider an alternative: for each version of something you need to support, create a separate class The class name could even include the version number of what it supports, to be really explicit about what it does Such classes are called Adapters [DP] Adapters implement a common interface and are responsible for functioning correctly with one (and usually only one) version of some code Adapters make it easy for client code to swap in support for one library or API version or another And programmers routinely rely on runtime information to configure their programs with the correct Adapter I refactor to Adapters fairly often I like Adapters because they let me decide how I want to communicate with other people's code In a fast-changing world, Adapters help me stay insulated from highly useful but rapidly changing APIs, such as those springing eternally from the open source world In some cases, Adapters may adapt too much For example, a client needs access to behavior on an adaptee, yet it cannot access that behavior because it only has access to the adaptee via an Adapter In that case, the Adapter must be redesigned to accommodate client needs Systems that depend on multiple versions of a component, library, or API tend to have a good deal of version-dependent logic scattered throughout the code (a sure sign of the Solution Sprawl smell, 43) While you wouldn't want to complicate a design by refactoring to Adapter too early, it's useful to apply this refactoring as soon as you find complexity, propagating conditionality, or a maintanance issue resulting from code written to handle multiple versions