package com.wrox.algorithms.iteration;import java.io.File;/** * Simple class to print out the contents of a directory tree. * */public final class RecursiveDirectoryTreePrinter {    /** The number of spaces to indent each recursion. */    private static final String SPACES = "  ";    /**     * Constructor marked private to prevent instantiation.     */    private RecursiveDirectoryTreePrinter() {    }    /**     * Program mainline.     *     * @param args Command-line arguments.     */    public static void main(String[] args) {        assert args != null : "args can't be null";        if (args.length != 1) {            System.err.println("Usage: RecursiveDirectoryTreePrinter <dir>");            System.exit(-1);        }        System.out.println("Recursively printing directory tree for: " + args[0]);        print(new File(args[0]), "");    }    /**     * Prints a list of files/directories with the given indentation.     *     * @param files The files/directories to print.     * @param indent The amount of indentation.     */    private static void print(Iterator files, String indent) {        assert files != null : "files can't be null";        for (files.first(); !files.isDone(); files.next()) {            print((File) files.current(), indent);        }    }    /**     * Prints a file or directory with the given indentation.     *     * @param file  The file or directory to print.     * @param indent The amount of indentation.     */    private static void print(File file, String indent) {        assert file != null : "file can't be null";        assert indent != null : "indent can't be null";        System.out.print(indent);        System.out.println(file.getName());        if (file.isDirectory()) {            print(new ArrayIterator(file.listFiles()), indent + SPACES);        }    }}