Use Visitor pattern to walk through file system : Visitor Pattern « Design Pattern « Java Tutorial

1. Java Tutorial
2. Design Pattern
3. Visitor Pattern

import java.io.File;

public class VisitorTester {
  public static void main(String[] args) {
    DirectoryNode node = new DirectoryNode(new File(".."));
    node.visit(new PrintVisitor());
  }
}

class PrintVisitor  {
  int level = 0;
  public void visitFileNode(FileNode node) {
    for (int i = 0; i < level; i++)
      System.out.print(" ");
    System.out.println(node.getFile().getName());
  }

  public void visitDirectoryNode(DirectoryNode node) {
    for (int i = 0; i < level; i++)
      System.out.print(" ");
    System.out.println(node.getDirectory().getName());
    level++;
    for (FileSystemNode c : node.getChildren())
      c.visit(this);
    level--;
  }
}

interface FileSystemNode {
  void visit(PrintVisitor v);
}

class FileNode implements FileSystemNode {
  public FileNode(File file) {
    this.file = file;
  }

  public File getFile() {
    return file;
  }

  public void visit(PrintVisitor v) {
    v.visitFileNode(this);
  }

  private File file;
}

class DirectoryNode implements FileSystemNode {
  public DirectoryNode(File directory) {
    this.directory = directory;
  }

  public void visit(PrintVisitor v) {
    v.visitDirectoryNode(this);
  }

  public File getDirectory() {
    return directory;
  }

  public FileSystemNode[] getChildren() {
    File[] files = directory.listFiles();
    FileSystemNode[] children = new FileSystemNode[files.length];
    for (int i = 0; i < files.length; i++) {
      File f = files[i];
      if (f.isDirectory())
        children[i] = new DirectoryNode(f);
      else
        children[i] = new FileNode(f);
    }
    return children;
  }

  private File directory;
}