Java Tip 98: Reflect on the Visitor design pattern

Implement visitors in Java, using reflection

Collections are commonly used in object-oriented programming and often raise code-related questions. For example, "How do you perform an operation across a collection of different objects?"

One approach is to iterate through each element in the collection and then do something specific to each element, based on its class. That can get pretty tricky, especially if you don't know what type of objects are in the collection. If you wanted to print out the elements in the collection, you could write a method like this:

public void messyPrintCollection(Collection collection) {
   Iterator iterator = collection.iterator()
   while (iterator.hasNext())
      System.out.println(iterator.next().toString())
}

That seems simple enough. You just call the Object.toString() method and print out the object, right? What if, for example, you have a vector of hashtables? Then things start to get more complicated. You must check the type of object returned from the collection:

public void messyPrintCollection(Collection collection) {
   Iterator iterator = collection.iterator()
   while (iterator.hasNext()) {
      Object o = iterator.next();
      if (o instanceof Collection)
         messyPrintCollection((Collection)o);
      else
         System.out.println(o.toString());
   }
}

OK, so now you have handled nested collections, but what about other objects that do not return the String that you need from them? What if you want to add quotes around String objects and add an f after Float objects? The code gets still more complex:

public void messyPrintCollection(Collection collection) {
   Iterator iterator = collection.iterator()
   while (iterator.hasNext()) {
      Object o = iterator.next();
      if (o instanceof Collection)
         messyPrintCollection((Collection)o);
      else if (o instanceof String)
         System.out.println("'"+o.toString()+"'");
      else if (o instanceof Float)
         System.out.println(o.toString()+"f");
      else
         System.out.println(o.toString());
   }
}

You can see that things can start to get intricate really fast. You don't want a piece of code with a huge list of if-else statements! How do you avoid that? The Visitor pattern comes to the rescue.

To implement the Visitor pattern, you create a Visitor interface for the visitor, and a Visitable interface for the collection to be visited. You then have concrete classes that implement the Visitor and Visitable interfaces. The two interfaces look like this:

public interface Visitor
{
   public void visitCollection(Collection collection);
   public void visitString(String string);
   public void visitFloat(Float float);
}
public interface Visitable
{
   public void accept(Visitor visitor);
}

For a concrete String, you might have:

public class VisitableString implements Visitable
{
   private String value;
   public VisitableString(String string) {
      value = string;
   }
   public void accept(Visitor visitor) {
      visitor.visitString(this);
   }
}

In the accept method, you call the correct visitor method for this type:

visitor.visitString(this)

That lets you implement a concrete Visitor as the following:

public class PrintVisitor implements Visitor
{
   public void visitCollection(Collection collection) {
      Iterator iterator = collection.iterator()
      while (iterator.hasNext()) {
      Object o = iterator.next();
      if (o instanceof Visitable)
         ((Visitable)o).accept(this);
   }
   public void visitString(String string) {
      System.out.println("'"+string+"'");
   }
   public void visitFloat(Float float) {
      System.out.println(float.toString()+"f");
   }
}

By then implementing a VisitableFloat class and a VisitableCollection class that each call the appropriate visitor methods, you get the same result as the messy if-else messyPrintCollection method but with a much cleaner approach. In visitCollection(), you call Visitable.accept(this), which in turn calls the correct visitor method. That is called a double-dispatch; the Visitor calls a method in the Visitable class, which calls back into the Visitor class.

Although you've cleaned up an if-else statement by implementing the visitor, you've still introduced a lot of extra code. You've had to wrap your original objects, String and Float, in objects implementing the Visitable interface. Although annoying, that is normally not a problem since the collections you are usually visiting can be made to contain only objects that implement the Visitable interface.

Still, it seems like a lot of extra work. Worse, what happens when you add a new Visitable type, say VisitableInteger? That is one major drawback of the Visitor pattern. If you want to add a new Visitable object, you have to change the Visitor interface and then implement that method in each of your Visitor implementation classes. You could use an abstract base class Visitor with default no-op functions instead of an interface. That would be similar to the Adapter classes in Java GUIs. The problem with that approach is that you need to use up your single inheritance, which you often want to save for something else, such as extending StringWriter. It would also limit you to only be able to visit Visitable objects successfully.

Luckily, Java lets you make the Visitor pattern much more flexible so you can add Visitable objects at will. How? The answer is by using reflection. With a ReflectiveVisitor, you only need one method in your interface:

public interface ReflectiveVisitor {
   public void visit(Object o);
}

OK, that was easy enough. Visitable can stay the same, and I'll get to that in a minute. For now, I'll implement PrintVisitor using reflection:

public class PrintVisitor implements ReflectiveVisitor {
   public void visitCollection(Collection collection)
   { ... same as above ... }
   public void visitString(String string)
   { ... same as above ... }
   public void visitFloat(Float float)
   { ... same as above ... }
   public void default(Object o)
   {
      System.out.println(o.toString());
   }
   public void visit(Object o) {
      // Class.getName() returns package information as well.
      // This strips off the package information giving us
      // just the class name
      String methodName = o.getClass().getName();
      methodName = "visit"+
                   methodName.substring(methodName.lastIndexOf('.')+1);
      // Now we try to invoke the method visit<methodName>
      try {
         // Get the method visitFoo(Foo foo)
         Method m = getClass().getMethod(methodName,
            new Class[] { o.getClass() });
         // Try to invoke visitFoo(Foo foo)
         m.invoke(this, new Object[] { o });
      } catch (NoSuchMethodException e) {
         // No method, so do the default implementation
         default(o);
      }
   }
}

Now you don't need the Visitable wrapper class. You can just call visit(), and it will dispatch to the correct method. One nice aspect is that visit() can dispatch however it sees fit. It doesn't have to use reflection -- it can use a totally different mechanism.

With the new PrintVisitor, you have methods for Collections, Strings, and Floats, but then you catch all the unhandled types in the catch statement. You'll expand upon the visit() method so that you can try all the superclasses as well. First, you'll add a new method called getMethod(Class c) that will return the method to invoke, which looks for a matching method for all the superclasses of Class c and then all the interfaces for Class c.

protected Method getMethod(Class c) {
   Class newc = c;
   Method m = null;
   // Try the superclasses
   while (m == null && newc != Object.class) {
      String method = newc.getName();
      method = "visit" + method.substring(method.lastIndexOf('.') + 1);
      try {
         m = getClass().getMethod(method, new Class[] {newc});
      } catch (NoSuchMethodException e) {
         newc = newc.getSuperclass();
      }
   }
   // Try the interfaces.  If necessary, you
   // can sort them first to define 'visitable' interface wins
   // in case an object implements more than one.
   if (newc == Object.class) {
      Class[] interfaces = c.getInterfaces();
      for (int i = 0; i < interfaces.length; i++) {
         String method = interfaces[i].getName();
         method = "visit" + method.substring(method.lastIndexOf('.') + 1);
         try {
            m = getClass().getMethod(method, new Class[] {interfaces[i]});
         } catch (NoSuchMethodException e) {}
      }
   }
   if (m == null) {
      try {
         m = thisclass.getMethod("visitObject", new Class[] {Object.class});
      } catch (Exception e) {
          // Can't happen
      }
   }
   return m;
}

It looks complicated, but it really isn't. Basically, you just look for methods based on the name of the class you have passed in. If you don't find one, you try its superclasses. Then if you don't find any of those, you try any interfaces. Lastly, you can just try visitObject() as a default.

Note that for the sake of those familiar with the traditional Visitor pattern, I've followed the same naming convention for the method names. However, as some of you may have noticed, it would be more efficient to name all the methods "visit" and let the parameter type be the differentiator. If you do that, however, make sure you change the main visit(Object o) method name to something like dispatch(Object o). Otherwise, you won't have a default method to fall back on, and you would need to cast to Object whenever you call visit(Object o) to assure the correct method calling pattern was followed.

Now, you modify the visit() method to take advantage of getMethod():

public void visit(Object object) {
   try {
     Method method = getMethod(getClass(), object.getClass());
     method.invoke(this, new Object[] {object});
   } catch (Exception e) { }
}

Now, your visitor object is much more powerful. You can pass in any arbitrary object and have some method that uses it. Plus, you gain the added benefit of having a default method visitObject(Object o) that can catch anything you don't specify. With a little more work, you can even add a method for visitNull().

I've kept the Visitable interface in there for a reason. Another side benefit of the traditional Visitor pattern is that it allows the Visitable objects to control navigation of the object structure. For example, if you had a TreeNode object that implemented Visitable, you could have an accept() method that traverses to its left and right nodes:

public void accept(Visitor visitor) {
   visitor.visitTreeNode(this);
   visitor.visitTreeNode(leftsubtree);
   visitor.visitTreeNode(rightsubtree);
}

So, with just one more modification to the Visitor class, you can allow for Visitable-controlled navigation:

public void visit(Object object) throws Exception
{
    Method method = getMethod(getClass(), object.getClass());
     method.invoke(this, new Object[] {object});
     if (object instanceof Visitable)
     {
          callAccept((Visitable) object);
     }
}
public void callAccept(Visitable visitable) {
   visitable.accept(this);
}

If you've implemented a Visitable object structure, you can keep the callAccept() method as is and use Visitable-controlled navigation. If you want to navigate the structure within the visitor, you just override the callAccept() method to do nothing.

The power of the Visitor pattern comes into play when using several different visitors across the same collection of objects. For example, I have an interpreter, an infix writer, a postfix writer, an XML writer, and a SQL writer working across the same collection of objects. I could easily write a prefix writer or a SOAP writer for the same collection of objects. In addition, those writers can gracefully work with objects they don't know about or, if I choose, they can throw an exception.

Conclusion

By using Java reflection, you can enhance the Visitor design pattern to provide a powerful way to operate on object structures, giving the flexibility to add new

Visitable

types as needed. I hope you are able to use that pattern somewhere in your coding travels.

Jeremy Blosser has been programming in Java for five years, during which he has worked for various software companies. He now works for a startup company, Software Instruments. You can visit Jeremy's Website at http://www.blosser.org.

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