该文章是阅读《图解设计模式》的学习笔记。书本链接https://www.ituring.com.cn/book/1811
在一个数据结构中保存着许多的元素,如果要对这些元素进行处理操作,通常的方法是直接将这些处理操作直接放在了数据结构的类中,每增加一种处理方式,就不得不修改表示数据结构的类,而使用Visitor模式,则可以将数据结构与处理分离出来。
Visitor抽象类代码:
package com.wen.Visitor;
public abstract class Visitor {
public abstract void visit(File file);
public abstract void visit(Directory directory);
}
Element接口代码:
package com.wen.Visitor;
public interface Element {
public abstract void accept(Visitor visitor);
}
Entry抽象类代码:
package com.wen.Visitor;
import java.util.Iterator;
public abstract class Entry implements Element{
public abstract String getName();
public abstract int getSize();
public Entry add(Entry entry) throws FileTreatmentException{
throw new FileTreatmentException();
}
public Iterator iterator()throws FileTreatmentException{
throw new FileTreatmentException();
}
public String toString(){
return getName()+"("+getSize()+")";
}
}
File类代码:
package com.wen.Visitor;
public class File extends Entry {
private String name;
private int size;
public File(String name, int size) {
this.name = name;
this.size = size;
}
@Override
public String getName() {
return name;
}
@Override
public int getSize() {
return size;
}
@Override
public void accept(Visitor visitor) {
visitor.visit(this);
}
}
Directory类代码:
package com.wen.Visitor;
import java.util.ArrayList;
import java.util.Iterator;
public class Directory extends Entry {
private String name;
private ArrayList dir = new ArrayList();
public Directory(String name) {
this.name = name;
}
@Override
public String getName() {
return name;
}
public int getSize(){
int size=0;
Iterator iterator = dir.iterator();
while(iterator.hasNext()){
Entry entry = (Entry)iterator.next();
size += entry.getSize();
}
return size;
}
public Entry add(Entry entry) {
dir.add(entry);
return this;
}
public Iterator iterator(){
return dir.iterator();
}
@Override
public void accept(Visitor visitor) {
visitor.visit(this);
}
}
ListVisitor类代码:
package com.wen.Visitor;
import java.util.Iterator;
public class ListVisitor extends Visitor {
private String currentdir = "";
public void visit(File file){
System.out.println(currentdir+"/"+file);
}
public void visit(Directory directory){
System.out.println(currentdir+"/"+directory);
String savedir = currentdir;
currentdir = currentdir+"/"+directory.getName();
Iterator iterator = directory.iterator();
while(iterator.hasNext()){
Entry entry = (Entry)iterator.next();
entry.accept(this);
}
currentdir = savedir;
}
}
FileTreatmentException异常处理类代码:
package com.wen.Visitor;
public class FileTreatmentException extends RuntimeException {
public FileTreatmentException() {
}
public FileTreatmentException(String msg){
super(msg);
}
}
Main入口类代码:
package com.wen.Visitor;
public class Main {
public static void main(String[] args) {
try {
System.out.println("创建管理员文件实体");
Directory rootdir = new Directory("root");
Directory bindir = new Directory("bin");
Directory tmpdir = new Directory("tmp");
Directory usrdir = new Directory("usr");
rootdir.add(bindir);
rootdir.add(tmpdir);
rootdir.add(usrdir);
bindir.add(new File("vi", 10000));
bindir.add(new File("latex", 20000));
rootdir.accept(new ListVisitor());
System.out.println("");
System.out.println("创建用户文件实体");
Directory yuki = new Directory("yuki");
Directory hanako = new Directory("hanako");
Directory tomura = new Directory("tomura");
usrdir.add(yuki);
usrdir.add(hanako);
usrdir.add(tomura);
yuki.add(new File("diary.html", 100));
yuki.add(new File("Composite.java", 200));
hanako.add(new File("memo.tex", 300));
tomura.add(new File("game.doc", 400));
tomura.add(new File("junk.mail", 500));
rootdir.accept(new ListVisitor());
} catch (FileTreatmentException e) {
e.printStackTrace();
}
}
}
该模式中的角色:
Visitor(访问者):声明访问具体元素的方法,对每一个具体的元素都会分别声明一个方法方法,如上面代码中的Visitor类。
ConcreteVisitor(具体的访问者):负责实现Visitor角色中定义的方法。如上面代码中的ListVisitor类。
Element(元素):表示Visitor角色的访问对象,声明了接受访问者的accept方法。如上面代码中的Element接口。
ConcreteElement(具体的元素):负责实现Element角色中定义的方法,如上面代码中的File和Directory类。
ObjectStructure(对象结构):负责处理Element角色的集合。负责处理Element角色的集合。如上面代码中的Directory类既扮演ConcreteElement角色也扮演ObjectStructure角色。