设计模式——访问者模式

访问者模式是一种行为模式，一般用于封装操作的变化，即对于一组对象（或一个对象）数据结构相对固定，但操作方法上有很多变化，这时候可以使用此设计模式；

角色：抽象的访问者角色，定义访问者针对此组对象需要实现的方法约束；

具体访问者角色，实现抽象访问者的约束

　　抽象被访问者角色，主要定义需要接受访问者的约束，一般是 void accept(Abstractvisitor visitor)

具体的被访问者角色。

访问者模式将数据结构和作用在数据结构上的操作解耦和；

下面看个示例;

/*
 * Copyright (c) 2017. panteng.Co.Ltd All rights reserved
 */

package com.pt.visitor;

/**
 * @description 抽象元素角色(被访问者角色)
 *      定义元素需要实现的接受访问者的方法
 * @author panteng
 * @date 17-3-6.
 */
public interface Element {
    void accept(Visitor visitor);
}

抽象元素类

/*
 * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved
 */

package com.pt.visitor;

/**
 * @description
 * @author panteng
 * @date 17-3-6.
 */
public class Number implements Element {
    double a;
    double b;

    public Number(){
    }

    public Number(double a, double b){
        this.a = a;
        this.b = b;
    }

    public void accept(Visitor visitor){
        visitor.visit(this);
    }
    public double getA(){
        return a;
    }
    public void setA(double a){
        this.a = a;
    }
    public double getB(){
        return b;
    }
    public void setB(double b){
        this.b = b;
    }
}

元素1

/*
 * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved
 */

package com.pt.visitor;

import java.util.List;

/**
 * @description
 * @author panteng
 * @date 17-3-6.
 */
public class MyCollection implements Element {
    List<Object> list1;
    List<Object> list2;

    public MyCollection(){
    }
    public MyCollection(List<Object> list1, List<Object> list2){
        this.list1 = list1;
        this.list2 = list2;
    }

    public void accept(Visitor visitor){
        visitor.visit(this);
    }

    public List<Object> getList1(){
        return list1;
    }
    public void setList1(List<Object> list1){
        this.list1 = list1;
    }
    public List<Object> getList2(){
        return list2;
    }
    public void setList2(List<Object> list2){
        this.list2 = list2;
    }
}

元素2

/*
 * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved
 */

package com.pt.visitor;

/**
 * @description 抽象访问者，定义访问操作接口
 * @author panteng
 * @date 17-3-6.
 */
public interface Visitor {
    void visit(Number num);
    void visit(MyCollection set);
}

抽象操作类

/*
 * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved
 */

package com.pt.visitor;

/**
 * @description
 * @author panteng
 * @date 17-3-6.
 */
public class Add implements Visitor {
    public void visit(MyCollection set){
        System.out.print(set.getList1() + " + " + set.getList2() + " = ");
        set.getList1().addAll(set.getList2());
        System.out.println(set.getList1());
    }
    public void visit(Number num){
        System.out.println(num.getA() + " + " + num.getB() + " = " + (num.getA() + num.getB()));
    }
}

操作类1

/*
 * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved
 */

package com.pt.visitor;

/**
 * @description
 * @author panteng
 * @date 17-3-6.
 */
public class Sub implements Visitor {
    public void visit(MyCollection set){
        System.out.print(set.getList1() + " - " + set.getList2() + " = ");
        set.getList1().removeAll(set.getList2());
        System.out.println(set.getList1());
    }
    public void visit(Number num){
        System.out.println(num.getA() + " - " + num.getB() + " = " + (num.getA() - num.getB()));
    }
}

操作类2

/*
 * Copyright (c) 2017. Xiaomi.Co.Ltd All rights reserved
 */

package com.pt.visitor;

import org.junit.Test;

import java.util.ArrayList;
import java.util.List;

/**
 * @description
 * @author panteng
 * @date 17-3-6.
 */
public class VisitorTest {
    @Test
    public void visitorTest(){
        Visitor addVisitor = new Add();
        Visitor subVisitor = new Sub();
        Number number = new Number(3, 9);

        List<Object> list1 = new ArrayList<Object>();
        list1.add(1);
        list1.add(2);
        list1.add(3);
        List<Object> list2 = new ArrayList<Object>();
        list2.add(3);
        list2.add(4);
        list2.add(5);
        MyCollection set = new MyCollection(list1, list2);

        number.accept(addVisitor);
        set.accept(addVisitor);

        number.accept(subVisitor);
        set.accept(subVisitor);
    }
}