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public class TagNode
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public String toString() {
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StringBuffer result = new StringBuffer(""); appendContentsTo(result); return resulttoString();
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} private void appendContentsTo(StringBuffer result) { writeOpenTagTo(result); writeChildrenTo(result); writeValueTo(result); writeEndTagTo(result); }
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private void writeOpenTagTo(StringBuffer result) { resultappend("<"); resultappend(name); resultappend(attributestoString()); resultappend(">"); } private void writeChildrenTo(StringBuffer result) { Iterator it = childreniterator(); while (ithasNext()) { TagNode node = (TagNode)itnext(); nodeappendContentsTo(result); } } private void writeValueTo(StringBuffer result) { if (!valueequals("")) resultappend(value); } private void writeEndTagTo(StringBuffer result) { resultappend("</"); resultappend(name); resultappend(">"); } }
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I compile, run my tests, and everything is good ThetoString() method is now very simple, while the appendContentsTo() method is a fine example of a Composed Method (see Compose Method, 123)
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Move Accumulation to Visitor
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A method accumulates information from heterogeneous classes
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Move the accumulation task to a Visitor that can visit each class to accumulate the information
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Ralph Johnson, one of the four authors of Design Patterns [DP], once observed, "Most of the time you don't need Visitor, but when you do need Visitor, you really need Visitor!" So when do you really need Visitor Let's review what Visitors are before answering that question A Visitor is a class that performs an operation on an object structure The classes that a Visitor visits are heterogeneous, which means they
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hold unique information and provide a specific interface to that information Visitors can easily interact with heterogeneous classes by means of double-dispatch This means that each of a set of classes accepts a Visitor instance as a parameter (via an "accept" method: accept(Visitor visitor)) and then calls back on the Visitor, passing itself to its corresponding visit method, as shown in the following diagram
Because the first argument passed to a Visitor's visit( ) method is an instance of a specific type, the Visitor can call type-specific methods on the instance without performing type-casting This makes it possible for Visitors to visit classes in the same hierarchy or different hierarchies The job of many real-world Visitors is to accumulate information The Collecting Parameter pattern is also useful in this role (see Move Accumulation to Collecting Parameter, 313) Like a Visitor, a Collecting Parameter may be passed to multiple objects to accumulate information from them The key difference lies in the ability to easily accumulate information from heterogeneous classes While Visitors have no trouble with this task due to double-dispatch, Collecting Parameters don't rely on double-dispatch, which limits their ability to gather diverse information from classes with diverse interfaces Now let's get back to the question: When do you really need a Visitor In general, you need a Visitor when you have numerous algorithms to run on the same heterogeneous object structure and no other solution is as simple or succinct as a Visitor For example, say you have three domain classes, none of which share a common superclass and all of which feature code for producing different XML representations
What's wrong with this design The main problem is that you have to add a new toXml method to each of these domain classes every time you have a new XML representation In addition, the toXml methods bloat the domain classes with representation code, which is better kept separate from the domain logic, particularly when you have a lot of it In the Mechanics section, I refer to the toXml methods as internal accumulation methods because they are internal to the classes used in the accumulation Refactoring to a Visitor changes the design as shown in the following diagram
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With this new design, the domain classes may be represented using whatever Visitor is appropriate Furthermore, the copious representation logic that once crowded the domain classes is now encapsulated in the appropriate Visitor Another case when a Visitor is needed is when you have numerous external accumulation methods Such methods typically use an Iterator [DP] and resort to type-casting heterogeneous objects to access specific information: public String extractText() while (nodeshasMoreNodes()) { Node node = nodesnextNode(); if (node instanceof StringNode) {
StringNode stringNode = (StringNode)node;
resultsappend(
stringNodegetText());
} else if (node instanceof LinkTag) {
LinkTag linkTag = (LinkTag)node;
if (isPreTag) resultsappend(
linkgetLinkText());
else resultsappend(
linkgetLink());
} else if }
Type-casting objects to access their specific interfaces is acceptable if it's not done frequently However, if this activity becomes frequent, it's worth considering a better design Would a Visitor provide a better solution Perhaps unless your heterogeneous classes suffer from the smell Alternative Classes with Different Interfaces [F] In that case, you could likely refactor the classes to have a common interface, thereby making it possible to accumulate information without type-casting or implementing a Visitor On the other hand, if you can't make heterogeneous classes look homogeneous by means of a common interface and you have numerous external accumulation methods, you can likely arrive at a better solution by refactoring to a Visitor The opening code sketch and the Example section show such a case Finally, there are times when you have neither an external nor an internal accumulation method, yet your design could be improved by replacing your existing code with a Visitor On the HTML Parser project, we once accomplished an information accumulation step by writing two new subclasses as shown in the figure on the following page
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After we studied the new subclasses we'd written, we realized that one Visitor could take the place of the subclasses and the code would be simpler and more succinct Yet we didn't jump to implementing a Visitor at that point; we felt that we needed further justification before taking on the nontrivial task of a Visitor refactoring We found that justification when we discovered several external accumulation methods in client code to the HTML Parser This illustrates the kind of thinking that ought to go into a decision to refactor to Visitor because such a refactoring is by no means a simple transformation If the set of classes your would-be Visitor must visit is growing frequently, it's generally advisable to avoid a Visitor solution because it involves writing an accept method on each new visitable class along with a corresponding visit method on the Visitor On the other hand, it's best to not follow this rule religiously When I considered refactoring to Visitor on the HTML Parser project, I found that the initial set of classes the Visitor would need to visit was too large and changed too frequently After further inquiry, I determined that only a subset of the classes actually needed to be visited; the rest of the classes could be visited by using the visit method for their superclass Some programmers object to the Visitor pattern for one reason or another before they get to know it For example, one programmer told me that he didn't like Visitor because it "breaks encapsulation" In other words, if a Visitor can't perform its work on a visitee because one or more of the visitee methods aren't public, the method(s) must be made public (thereby breaking encapsulation) to let the Visitor do its work True Yet many Visitor implementations require no visibility changes on visitees (see the upcoming Example section) and, even if a few visibility changes are required, the price you pay for compromising a visitee's encapsulation may be far lower than the price you pay to live with a non-Visitor solution Another objection raised against the Visitor pattern is that it adds too much complexity or obscurity to code One programmer said, "Looking at the visit loop tells you nothing about what is being performed" The "visit loop" is code that iterates over visitees in an object structure and passes the Visitor to each one of them While it's true that a visit loop reveals little about what concrete Visitors actually do, it's clear what the visit loop does if you understand the Visitor pattern So the complexity or obscurity of a Visitor implementation depends a lot on an individual's or team's comfort level with the pattern In addition, if a Visitor is really needed in a system, it will make overly complex or obscure code simpler The double-edged sword of the Visitor pattern is its power and sophistication When you need a Visitor, you really need one, as Ralph says Unfortunately, too many programmers feel the need to use Visitor for the wrong reasons, like showing off or because they're still "patterns happy" Always consider simpler solutions before refactoring to Visitor, and use this pattern most judiciously