设计模式学习—访问者模式(Visitor Design Pattern)

曾经终败给现在 2022-06-09 07:28 109阅读 0赞

# 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 #

![Center][]

###### 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 #

[Center]: /images/20220609/c523ed52b307407eb6df95330e4eab52.png