3. 行为型模式 (9) 访问者模式

3. 行为型模式 (9) 访问者模式

访问者模式（Visitor Pattern）中元素的执行算法可以随着访问者改变而改变。

元素对象已接受访问者对象，这样访问者对象就可以处理元素对象上的操作。

优点：
1. 符合单一职责原则。
2. 优秀的扩展性。
3. 灵活性。

缺点：
1. 具体元素对访问者公布细节，违反了迪米特原则。
2. 具体元素变更比较困难。
3. 违反了依赖倒置原则，依赖了具体类，没有依赖抽象。

1. 访问者模式

package com.andrew.pattern0309.visitor.model01;
public interface ComputerPart {
    public void accept(ComputerPartVisitor computerPartVisitor);
}

package com.andrew.pattern0309.visitor.model01;
public class Keyboard implements ComputerPart {
    @Override
    public void accept(ComputerPartVisitor computerPartVisitor) {
        computerPartVisitor.visit(this);
    }
}

package com.andrew.pattern0309.visitor.model01;
public class Monitor implements ComputerPart {
    @Override
    public void accept(ComputerPartVisitor computerPartVisitor) {
        computerPartVisitor.visit(this);
    }
}

package com.andrew.pattern0309.visitor.model01;
public class Mouse implements ComputerPart {
    @Override
    public void accept(ComputerPartVisitor computerPartVisitor) {
        computerPartVisitor.visit(this);
    }
}

package com.andrew.pattern0309.visitor.model01;
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);
    }
}

package com.andrew.pattern0309.visitor.model01;
public interface ComputerPartVisitor {
    public void visit(Computer computer);
    public void visit(Mouse mouse);
    public void visit(Keyboard keyboard);
    public void visit(Monitor monitor);
}

package com.andrew.pattern0309.visitor.model01;
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.");
    }
}

package com.andrew.pattern0309.visitor.model01;
/**
 * 1. 访问者模式
 * 
 * @author andrew
 * @date 2018/08/20
 */
public class Client {
    public static void main(String[] args) {
        ComputerPart computer = new Computer();
        computer.accept(new ComputerPartDisplayVisitor());
    }
}
运行结果：
Displaying Mouse.
Displaying Keyboard.
Displaying Monitor.
Displaying Computer.

2. 访问者模式

package com.andrew.pattern0309.visitor.model02;
public interface Element {
    public abstract void accept(IVisitor visitor);
    public abstract void doSomething();
}

package com.andrew.pattern0309.visitor.model02;
public class ConcreteElement1 implements Element {
    public void accept(IVisitor visitor) {
        visitor.visit(this);
    };
    public void doSomething() {
        System.out.println("这是元素1");
    }
}

package com.andrew.pattern0309.visitor.model02;
public class ConcreteElement2 implements Element {
    public void accept(IVisitor visitor) {
        visitor.visit(this);
    };
    public void doSomething() {
        System.out.println("这是元素2");
    }
}

package com.andrew.pattern0309.visitor.model02;
public interface IVisitor {
    public void visit(ConcreteElement1 el1);
    public void visit(ConcreteElement2 el2);
}

package com.andrew.pattern0309.visitor.model02;
public class Visitor implements IVisitor {
    @Override
    public void visit(ConcreteElement1 el1) {
        el1.doSomething();
    }
    @Override
    public void visit(ConcreteElement2 el2) {
        el2.doSomething();
    }
}

package com.andrew.pattern0309.visitor.model02;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
public class ObjectStruture {
    public static List<Element> getList() {
        List<Element> list = new ArrayList<Element>();
        Random ran = new Random();
        for (int i = 0; i < 10; i++) {
            int a = ran.nextInt(100);
            if (a > 50) {
                list.add(new ConcreteElement1());
            } else {
                list.add(new ConcreteElement2());
            }
        }
        return list;
    }
}

package com.andrew.pattern0309.visitor.model02;
import java.util.List;
/**
 * 2. 访问者模式
 * 
 * @author andrew
 * @date 2018/08/20
 */
public class Client {
    public static void main(String[] args) {
        List<Element> list = ObjectStruture.getList();
        for (Element e: list) {
            e.accept(new Visitor());
        }
    }
}
运行结果：
这是元素1
这是元素1
这是元素1
这是元素1
这是元素2
这是元素2
这是元素1
这是元素2
这是元素1
这是元素1