Design pattern ~ Visitor ~

1.First of all

Here is a summary of the ** Visitor pattern ** in the GoF design pattern.

2. What is the Visitor pattern?

--The English word Visitor means ** visitor **. --The Visitor pattern is a ** method that separates data structure and processing **. --Prepare a visitor class that walks through the data structure and let the visitor class handle the process. Then, when you want to add a new process, you can create a new visitor. And the data structure just needs to accept the visitor. --The GoF design patterns are classified as ** behavioral design patterns **.

3. Sample class diagram

4. Sample program

A program that displays a list of directories and files.

4-1. Element interface

An interface that represents a data structure that accepts instances of the Visitor class.

Element.java

public interface Element {
	public abstract void accept(Visitor v);
}

4-2. Entry class

This is the base class for File and Directory. Implement the Element interface.

Entry.java

public abstract class Entry implements Element {

	public abstract String getName();

	public String toString() {
		return getName();
	}
}

4-3. File class

A class that represents a file. It will be a host for Visitors.

File.java

public class File extends Entry {

	private String name;

	public File(String name) {
		this.name = name;
	}

	public String getName() {
		return name;
	}

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

4-4. Directory class

A class that represents a directory. It will be a host for Visitors.

Directory.java

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

public class Directory extends Entry {

	private String name;
	private ArrayList<Entry> dir = new ArrayList<Entry>();

	public Directory(String name) {
		this.name = name;
	}

	public String getName() {
		return name;
	}

	public Entry add(Entry entry) {
		dir.add(entry);
		return this;
	}

	public Iterator<Entry> iterator() {
		return dir.iterator();
	}

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

4-5. Visitor class

An abstract class that represents a visitor to a file or directory.

Visitor.java

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

4-6. ListVisitor class

A class that displays a list of files and directories.

ListVisitor.java

import java.util.Iterator;

public class ListVisitor extends Visitor {

	//Directory name currently in interest
	private String currentdir = "";

	//Called when visiting a file
	public void visit(File file) {
		System.out.println(currentdir + "/" + file);
	}

	//Called when visiting a directory
	public void visit(Directory directory) {
		System.out.println(currentdir + "/" + directory);
		String savedir = currentdir;
		currentdir = currentdir + "/" + directory.getName();
		Iterator<Entry> it = directory.iterator();
		while (it.hasNext()) {
			Entry entry = (Entry) it.next();
			entry.accept(this);
		}
		currentdir = savedir;
	}
}

4-7. Main class

This class performs the main processing.

Main.java

public class Main {
	public static void main(String[] args) {

		Directory workspaceDir = new Directory("workspace");
		Directory compositeDir = new Directory("Visitor");
		Directory testDir1 = new Directory("test1");
		Directory testDir2 = new Directory("test2");
		workspaceDir.add(compositeDir);
		workspaceDir.add(testDir1);
		workspaceDir.add(testDir2);

		File element = new File("Element.java");
		File entity = new File("Entity.java");
		File file = new File("file.java");
		File directory = new File("Directory.java");
		File visitor = new File("Visitor.java");
		File listVisitor = new File("ListVisitor.java");
		File main = new File("main.java");
		compositeDir.add(element);
		compositeDir.add(entity);
		compositeDir.add(file);
		compositeDir.add(directory);
		compositeDir.add(visitor);
		compositeDir.add(listVisitor);
		compositeDir.add(main);

		workspaceDir.accept(new ListVisitor());
	}
}

4-8. Execution result

/workspace
/workspace/Visitor
/workspace/Visitor/Element.java
/workspace/Visitor/Entity.java
/workspace/Visitor/file.java
/workspace/Visitor/Directory.java
/workspace/Visitor/Visitor.java
/workspace/Visitor/ListVisitor.java
/workspace/Visitor/main.java
/workspace/test1
/workspace/test2

5. Benefits

The Visitor pattern only complicates the process, and I feel that "If you need iterative processing, why not write a loop process in the data structure?" The purpose of the Visitor pattern is ** to separate data structures and processing **. A data structure organizes elements as a set and connects elements. Preserving that structure is one thing, and writing a process based on that structure is another. The Visitor role (List Visitor) can be developed independently of the Acceptor role (File, Directory). In other words, the Visitor pattern increases the independence of the acceptor (File, Directory) class as a ** part **. If the content of the process is implemented as a method of the File class or Directory class, the File class or Directory class will have to be modified every time a new process is added and the function is extended.

6. GitHub

• https://github.com/i-tanaka730/design_pattern

7. List of design patterns

-** GoF design pattern summary **

8. Reference

This article and sample program were created based on the following books.

-** Introduction to design patterns learned in Java language **

It was very easy to understand and I learned a lot. Thank you. The detailed explanations of the design patterns and sample programs are written, so please take a look at the books as well.