객체 구조를 돌아다니면서 동일한 조작을 반복해서 적용하기. 이 때, '객체 구조'와 '처리(Algorithm)'을 분리하는 것이 핵심. 개방-폐쇄 원칙(OCP, Open-Closed Principle)¹⁾을 적용한 방법중 하나.

• 언제 사용하는가?
  • 데이터 구조 안에 많은 요소가 저장되어 있고, 그 각 요소에 대해서 다른 종류의 처리(Algorithm)가 필요한 경우
  • 이로써 얻는 이점으로 객체 구조와 처리 역할을 독립적으로 확장 가능!

• 시나리오
  • Composite Pattern의 예제인 “파일 시스템” 탐색을 이용. 여기서 처리 알고리즘을 분리시킨다.

Method Overloading을 이용하여 각각의 처리할 Logic 수행하는 역할.

package visitor;

import element.objectstructure.concrete.Directory;
import element.objectstructure.concrete.File;

public abstract class Visitor {
	public abstract void visit(File file);
	public abstract void visit(Directory directory);
}

package visitor.concrete;

import java.util.Iterator;

import element.objectstructure.Entry;
import element.objectstructure.concrete.Directory;
import element.objectstructure.concrete.File;
import visitor.Visitor;

public class ListVisitor extends Visitor {

	private String currentDirectoryName = "";
	
	@Override
	public void visit(File file) {
		System.out.println(currentDirectoryName + " / " + file);
	}

	@Override
	public void visit(Directory directory) {
		System.out.println(currentDirectoryName + " / " + directory);
		
		String tempDirectoryName = currentDirectoryName;
		
		currentDirectoryName = new StringBuilder(currentDirectoryName).append("/").append(directory.getName()).toString();
		Iterator<Entry> it = directory.iterator();
		while(it.hasNext()) {
			it.next().accept(this);
		}
		
		currentDirectoryName = tempDirectoryName; 
	}
}

방문할 객체를 지정하는 역할. accept()로 요청하여 객체 각각의 처리 Logic 수행.

package element;

import visitor.Visitor;

public interface Element {
	void accept(Visitor visitor);
}

package element.objectstructure;

import java.util.Iterator;

import element.Element;
import element.objectstructure.exception.FileTreatmentException;

public abstract class Entry implements Element {
	public abstract String getName();
	public abstract int getSize();
	
	/* Directory에서만 유효 */
	public Entry add(Entry entry) throws FileTreatmentException {
		throw new FileTreatmentException();
	}
	
	/* Visitor 역할에 필요한 정보 */
	/* Directory에서만 유효 */
	public Iterator<Entry> iterator() throws FileTreatmentException {
		throw new FileTreatmentException();
	}
	
	@Override
	public String toString() {
		StringBuilder sb = new StringBuilder();
		sb.append(getName());
		sb.append(" (");
		sb.append(getSize());
		sb.append(") ");
		
		return sb.toString();
	}
}

package element.objectstructure.concrete;

import element.objectstructure.Entry;
import visitor.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 void accept(Visitor visitor) {
		visitor.visit(this);
	}

	@Override
	public String getName() {
		return name;
	}

	@Override
	public int getSize() {
		return size;
	}

}

package element.objectstructure.concrete;

import java.util.ArrayList;
import java.util.Iterator;

import element.objectstructure.Entry;
import element.objectstructure.exception.FileTreatmentException;
import visitor.Visitor;

public class Directory extends Entry {

	private String name;
	private ArrayList<Entry> directories = new ArrayList<>();
	
	public Directory(String name) {
		this.name = name;
	}

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

	@Override
	public String getName() {
		return name;
	}

	@Override
	public int getSize() {
		return directories.size();
	}
	
	@Override
	public Entry add(Entry entry) throws FileTreatmentException {
		directories.add(entry);
		return this;
	}
	
	@Override
	public Iterator<Entry> iterator() throws FileTreatmentException {
		return directories.iterator();
	}
}

package element.objectstructure.exception;

public class FileTreatmentException extends RuntimeException {
	public FileTreatmentException() {
	}
	
	public FileTreatmentException(String message) {
		super(message);
	}
}

import element.objectstructure.concrete.Directory;
import element.objectstructure.concrete.File;
import visitor.concrete.ListVisitor;

public class Client {
    public static void main(String[] args) {
    	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("sub1", 10000));
        bindir.add(new File("sub2", 20000));
        
        rootdir.accept(new ListVisitor());
    }
}

/*
 / root (3) 
/root / bin (2) 
/root/bin / vi (10000) 
/root/bin / latex (20000) 
/root / tmp (0) 
/root / usr (0) 
*/