question: public FilesystemWalker ( final File startingDirectory ) { if ( startingDirectory == null ) { throw new IllegalArgumentException ( "startingDirectory must be specified" ) ; } if ( ! startingDirectory . isDirectory ( ) ) { final String messageFormat = "the specified starting directory [{0}] is not a directory" ; final String message = MessageFormat . format ( messageFormat , startingDirectory . getAbsolutePath ( ) ) ; throw new IllegalArgumentException ( message ) ; } this . startingDirectory = startingDirectory ; }
target:  Create a new FilesystemWalker that will walk beginning at the given directory.