[Design Mode] - Visitor Visitor Pattern

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　　Preface: [ Mode Overview ] ---------- by xingoo

　　Intention mode

　　For an object or group of objects, different visitors, produced different results, operations are different. At this point, the visitor is a typical application of the model.

　　Scenarios

　　A different sub-classes, dependent on various other objects

　　2 requires a set of objects, many unrelated operations, do not want to be in the class now these methods

　　3 defined classes rarely changes, but the operation was performed to change frequently.

　　Mode structure

　　Context environment role

class Context{
    List<Node> list = new ArrayList();
    public void add(Node node) {
        list.add(node);
    }
    public void visit(Visitor visitor) {
        for(Node node : list){
            node.accept(visitor);
        }
    }
}

　　Visitor role of visitors

interface Visitor{
    public void visit(NodeA nodeA);
    public void visit(NodeB nodeB);
}
class VisitA implements Visitor{
    public void visit(NodeA nodeA){
        System.out.println("***visitA***");
        nodeA.action();
    }
    public void visit(NodeB nodeB){
        System.out.println("***visitA***");
        nodeB.action();
    }
}
class VisitB implements Visitor{
    public void visit(NodeA nodeA){
        System.out.println("***visitB***");
        nodeA.action();
    }
    public void visit(NodeB nodeB){
        System.out.println("***visitB***");
        nodeB.action();
    }
}

　　Node is accessed role

interface Node{
    public void accept(Visitor visitor);
}
class NodeA implements Node{
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
    public void action(){
        System.out.println("NodeA visited");
    }
}
class NodeB implements Node{
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
    public void action(){
        System.out.println("NodeB visited");
    }
}

　　All codes

 1 package com.xingoo.test.design.visitor;
 2 
 3 import java.util.ArrayList;
 4 import java.util.List;
 5 
 6 interface Visitor{
 7     public void visit(NodeA nodeA);
 8     public void visit(NodeB nodeB);
 9 }
10 class VisitA implements Visitor{
11     public void visit(NodeA nodeA){
12         System.out.println("***visitA***");
13         nodeA.action();
14     }
15     public void visit(NodeB nodeB){
16         System.out.println("***visitA***");
17         nodeB.action();
18     }
19 }
20 class VisitB implements Visitor{
21     public void visit(NodeA nodeA){
22         System.out.println("***visitB***");
23         nodeA.action();
24     }
25     public void visit(NodeB nodeB){
26         System.out.println("***visitB***");
27         nodeB.action();
28     }
29 }
30 interface Node{
31     public void accept(Visitor visitor);
32 }
33 class NodeA implements Node{
34     public void accept(Visitor visitor) {
35         visitor.visit(this);
36     }
37     public void action(){
38         System.out.println("NodeA visited");
39     }
40 }
41 class NodeB implements Node{
42     public void accept(Visitor visitor) {
43         visitor.visit(this);
44     }
45     public void action(){
46         System.out.println("NodeB visited");
47     }
48 }
49 class Context{
50     List<Node> list = new ArrayList();
51     public void add(Node node) {
52         list.add(node);
53     }
54     public void visit(Visitor visitor) {
55         for(Node node : list){
56             node.accept(visitor);
57         }
58     }
59 }
60 public class Client {
61     private static Context ctx = new Context();
62     public static void main(String[] args) {
63         ctx.add(new NodeA());
64         ctx.add(new NodeB());
65         ctx.visit(new VisitA());
66         ctx.visit(new VisitB());
67     }
68 }

View Code

 

　　operation result

***visitA***
NodeA visited
***visitA***
NodeB visited
***visitB***
NodeA visited
***visitB***
NodeB visited

 

Reproduced in: https: //my.oschina.net/u/204616/blog/544979