谁说visitor不可以添加新元素?

这个visitor就可以随便添加,利用反射就可以做到了.
public abstract class Visitor {

final public void visit(Object object) {
Method method = getVisitMethod(object);
try {
method.invoke(this, new Object[] { object });
} catch (Exception e) {
String message = "invokeing method failed:visit("
+ object.getClass().getName() + ")";
throw new RuntimeException(message, e);
}
}

private Map visitMethods = new HashMap();

private Method getVisitMethod(Object object) {
//return (Method) visitMethods.get(object.getClass());
Iterator it = visitMethods.keySet().iterator();
for (; it.hasNext();) {
Class clazz = (Class) it.next();
if (clazz.isAssignableFrom(object.getClass())) {
return (Method) visitMethods.get(clazz);
}
}
throw new RuntimeException("method: visit(" + object.getClass().getName()
+ ") undefined in the class");
}

private Method visitMethod;

private void initVisitMethod() {
Method[] methods = getClass().getDeclaredMethods();
for (int i = 0; i < methods.length; i++) {
if (methods.getName().equals("visit")) {
Class[] paramTypes = methods.getParameterTypes();
if (paramTypes.length == 1) {
visitMethods.put(paramTypes[0], methods);
}
}
}
}

{
initVisitMethod();
}
}

具体的Visitor
public class PrintVisitor extends Visitor{

public void visit(Float f){
System.out.println("float:"+f);
}
public void visit(Date date){
System.out.println("date:"+date);
}
public void visit(String str){
System.out.println("string:"+str);
}
}

public class PrintVisitor2 extends Visitor{

public void visit(String str){
System.out.println("string2:"+str);
}
public void visit(List list){
System.out.println("list2:"+list);
}
}

public void testVisit(){
ArrayList list = new ArrayList();
list.add("a");
list.add("b");
Visitor v = new PrintVisitor();
v.visit("hello");
v.visit(new Date());
Visitor v1 = new PrintVisitor2();
v1.visit("hello");
v1.visit(list);
}