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visitor

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Visitor

定义

表示一个作用于某对象结构中的各元素的操作

可以在不改变各元素的类的前提下定义作用于这些元素的新操作

栗子

递归遍历某个文件夹下的所有文件夹和文件

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package eternal.fire;

import java.io.File;

public interface Visitor {
    //visit a directory
    void visitDir(File dir);

    //visit a file
    void visitFile(File file);
}
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package eternal.fire;

import java.io.File;
import java.util.Objects;

public class FileStructure {
    private File path;

    public FileStructure(File path) {
        this.path = path;
    }

    public void handle(Visitor visitor) {
        scan(path, visitor);
    }

    private void scan(File file, Visitor visitor) {
        if (file.isDirectory()) {
            visitor.visitDir(file);
            for (File f : Objects.requireNonNull(file.listFiles())) {
                scan(f, visitor);
            }
        } else if (file.isFile()) {
            visitor.visitFile(file);
        }
    }
}
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package eternal.fire;

import java.io.File;

public class JavaFileVisitor implements Visitor {

    @Override
    public void visitDir(File dir) {
        System.out.println("I'm visiting the directory:" + dir);
    }

    @Override
    public void visitFile(File file) {
        System.out.println("I'm visiting the file:" + file);
    }
}
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package eternal.fire;

import java.io.File;

public class ClassFileVisitor implements Visitor{

    @Override
    public void visitDir(File dir) {

    }

    @Override
    public void visitFile(File file) {
        if (file.getName().endsWith(".class")) {
            System.out.println("I'm visiting the class file");
        }
    }
}
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package eternal.fire;

import java.io.File;

public class Main {
    public static void main(String[] args) {
        FileStructure fileStructure = new FileStructure(new File("."));
        fileStructure.handle(new JavaFileVisitor());
        fileStructure.handle(new ClassFileVisitor());
    }
}

输出

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I'm visiting the directory:.
I'm visiting the directory:.\.idea
I'm visiting the file:.\.idea\.gitignore
I'm visiting the file:.\.idea\misc.xml
I'm visiting the file:.\.idea\modules.xml
I'm visiting the file:.\.idea\workspace.xml
I'm visiting the directory:.\out
I'm visiting the directory:.\out\production
I'm visiting the directory:.\out\production\Visitor
I'm visiting the directory:.\out\production\Visitor\eternal
I'm visiting the directory:.\out\production\Visitor\eternal\fire
I'm visiting the file:.\out\production\Visitor\eternal\fire\ClassFileVisitor.class
I'm visiting the file:.\out\production\Visitor\eternal\fire\FileStructure.class
I'm visiting the file:.\out\production\Visitor\eternal\fire\JavaFileVisitor.class
I'm visiting the file:.\out\production\Visitor\eternal\fire\Main.class
I'm visiting the file:.\out\production\Visitor\eternal\fire\Visitor.class
I'm visiting the directory:.\src
I'm visiting the directory:.\src\eternal
I'm visiting the directory:.\src\eternal\fire
I'm visiting the file:.\src\eternal\fire\ClassFileVisitor.java
I'm visiting the file:.\src\eternal\fire\FileStructure.java
I'm visiting the file:.\src\eternal\fire\JavaFileVisitor.java
I'm visiting the file:.\src\eternal\fire\Main.java
I'm visiting the file:.\src\eternal\fire\Visitor.java
I'm visiting the file:.\Visitor.iml
I'm visiting the class file:.\out\production\Visitor\eternal\fire\ClassFileVisitor.class
I'm visiting the class file:.\out\production\Visitor\eternal\fire\FileStructure.class
I'm visiting the class file:.\out\production\Visitor\eternal\fire\JavaFileVisitor.class
I'm visiting the class file:.\out\production\Visitor\eternal\fire\Main.class
I'm visiting the class file:.\out\production\Visitor\eternal\fire\Visitor.class

Process finished with exit code 0

优势

新增操作而不必对现有的对象结构做任何改动