一、我的理解
访问者模式允许外部类来改变本类的行为。本类会依据“访问”的外部类的类型不同而做出相应的反应。
二、实现方式
主体类提供对外接口，允许访客调用，而对于访客规定统一接口，执行不同的方法。
三、实例
Java实例
ComputerPart接口：规定主体类统一的对外接口
package com.study.dp.visitor;
public interface ComputerPart {
public void accept(ComputerPartVisitor computerPartVisitor);
}
ComputerPartVisitor接口：规定访客的统一行为
package com.study.dp.visitor;
public interface ComputerPartVisitor {
public void visit(Computer computer);
public void visit(Mouse mouse);
public void visit(Keyboard keyboard);
public void visit(Monitor monitor);
}
Keyboard类：被访问主体一
package com.study.dp.visitor;
public class Keyboard implements ComputerPart {
@Override
public void accept(ComputerPartVisitor computerPartVisitor) {
computerPartVisitor.visit(this);
}
}
Monitor类：被访问主体二
package com.study.dp.visitor;
public class Monitor implements ComputerPart {
@Override
public void accept(ComputerPartVisitor computerPartVisitor) {
computerPartVisitor.visit(this);
}
}
Mouse类：可以被访问的主体三
package com.study.dp.visitor;
public class Mouse implements ComputerPart {
@Override
public void accept(ComputerPartVisitor computerPartVisitor) {
computerPartVisitor.visit(this);
}
}
Computer类：可以被访问的主体四
package com.study.dp.visitor;
public class Computer implements ComputerPart {
ComputerPart[] parts;
public Computer() {
parts = new ComputerPart[] {new Mouse(), new Keyboard(), new Monitor()};
}
@Override
public void accept(ComputerPartVisitor computerPartVisitor) {
for (int i = 0; i < parts.length; i++) {
parts[i].accept(computerPartVisitor);
}
computerPartVisitor.visit(this);
}
}
ComputerPartDisplayVisitor类：实际的访客
package com.study.dp.visitor;
public class ComputerPartDisplayVisitor implements ComputerPartVisitor {
@Override
public void visit(Computer computer) {
System.out.println("Displaying Computer.");
}
@Override
public void visit(Mouse mouse) {
System.out.println("Displaying Mouse.");
}
@Override
public void visit(Keyboard keyboard) {
System.out.println("Displaying Keyboard.");
}
@Override
public void visit(Monitor monitor) {
System.out.println("Displaying Monitor.");
}
}
Demo类：测试类
package com.study.dp.visitor;
public class Demo {
public static void main(String[] args) {
ComputerPart computer = new Computer();
computer.accept(new ComputerPartDisplayVisitor());
}
}
四、应用场景
