在Wikipedia
sample和GoF书中,访问者模式的使用是通过在某个接受者上调用accept方法来启动的.但为什么会这样呢？为什么我们不能开始使用所需的acceptor作为参数调用visit方法？我们仍然可以使访问者行为依赖于两种类型 – 访问者和接受者(双重调度) – 并且我们可以消除冗余呼叫(在我看来).
这是用于说明这一点的示例代码：
public interface Visitor {
void visit(AcceptorA acceptor);
void visit(AcceptorB acceptor);
}
//
// Visitor which sings
//
class SingingVisitor implements Visitor {
public void visit(AcceptorA acceptor) {
System.out.println("sing A");
}
public void visit(AcceptorB acceptor) {
System.out.println("sing B");
}
}
//
// Visitor which talks
//
class TalkingVisitor implements Visitor {
public void visit(AcceptorA acceptor) {
System.out.println("talk A");
}
public void visit(AcceptorB acceptor) {
System.out.println("talk B");
}
}
//
// Acceptor subclasses
//
class AcceptorA implements BaseAcceptor {
}
class AcceptorB implements BaseAcceptor {
}
//
// Launcher class
//
class VisitorMain {
public static void main(String[] args) {
Visitor v = new TalkingVisitor();
AcceptorA a = new AcceptorA();
AcceptorB b = new AcceptorB();
v.visit(a);
v.visit(b);
v = new SingingVisitor();
v.visit(a);
v.visit(b);
}
}
解决方法
考虑：
class House implements HouseAcceptor {
HouseAcceptor kitchen;
HouseAcceptor livingRoom;
void accept(HouseVisitor visitor) {
visitor.visit(this);
kitchen.accept(visitor);
livingRoom.accept(visitor);
}
}
class Kitchen implements HouseAcceptor {
void accept(HouseVisitor visitor) {
visitor.visit(this);
}
}
class LivingRoom implements HouseAcceptor {
void accept(HouseVisitor visitor) {
visitor.visit(this);
}
}
class SpeakingHouseVisitor implements HouseVisitor {
void visit(HouseAcceptor acceptor) {
System.out.println("Inside a HouseAcceptor");
}
void visit(House acceptor) {
System.out.println("Inside a House");
}
void visit(Kitchen acceptor) {
System.out.println("Inside a Kitchen");
}
void visit(LivingRoom acceptor) {
System.out.println("Inside a LivingRoom");
}
}
...
HouseAcceptor acceptor = new House();
HouseVisitor visitor = new SpeakingHouseVisitor();
...
// Doing it your way
visitor.visit(acceptor);
// Output: Inside a HouseAcceptor
// Doing it the right way
acceptor.accept(visitor);
// Output:
// Inside a House
// Inside a Kitchen
// Inside a LivingRoom
请注意,如果按照自己的方式执行,接受器的运行时类型将没有区别：将使用静态类型.通过执行双重调度,您可以确保使用两种运行时类型.