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copied!<p>There is the "Visitor Pattern With Default", in which you do the visitor pattern as normal but then define an abstract class that implements your <code>IShapeVisitor</code> class by delegating everything to an abstract method with the signature <code>visitDefault(IShape)</code>.</p> <p>Then, when you define a visitor, extend this abstract class instead of implementing the interface directly. You can override the <code>visit</code>* methods you know about at that time, and provide for a sensible default. However, if there really isn't any way to figure out sensible default behavior ahead of time, you should just implement the interface directly.</p> <p>When you add a new <code>IShape</code> subclass, then, you fix the abstract class to delegate to its <code>visitDefault</code> method, and every visitor that specified a default behavior gets that behavior for the new <code>IShape</code>.</p> <p>A variation on this if your <code>IShape</code> classes fall naturally into a hierarchy is to make the abstract class delegate through several different methods; for example, an <code>DefaultAnimalVisitor</code> might do:</p> <pre><code>public abstract class DefaultAnimalVisitor implements IAnimalVisitor { // The concrete animal classes we have so far: Lion, Tiger, Bear, Snake public void visitLion(Lion l) { visitFeline(l); } public void visitTiger(Tiger t) { visitFeline(t); } public void visitBear(Bear b) { visitMammal(b); } public void visitSnake(Snake s) { visitDefault(s); } // Up the class hierarchy public void visitFeline(Feline f) { visitMammal(f); } public void visitMammal(Mammal m) { visitDefault(m); } public abstract void visitDefault(Animal a); } </code></pre> <p>This lets you define visitors that specify their behavior at whatever level of specificity you wish.</p> <p>Unfortunately, there is no way to avoid doing something to specify how visitors will behave with a new class - either you can set up a default ahead of time, or you can't. (See also the second panel of <a href="http://xkcd.com/568/" rel="noreferrer">this cartoon</a> )</p>

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