Chain of Responsibility Pattern

class UnixLister {
    private nextInLine
    UnixLister(next) { nextInLine = next }
    def listFiles(dir) {
        if (System.getProperty("'os.name"') == "'Linux"')
            println "ls $dir".execute().text
        else
            nextInLine.listFiles(dir)        
    }
}

class WindowsLister {
    private nextInLine
    WindowsLister(next) { nextInLine = next }
    def listFiles(dir) {
        if (System.getProperty("'os.name"') == "'Windows XP"')
            println "cmd.exe /c dir $dir".execute().text
        else
            nextInLine.listFiles(dir)        
    }
}

class DefaultLister {
    def listFiles(dir) {
        new File(dir).eachFile{ f -> println f }
    }
}

def lister = new UnixLister(new WindowsLister(new DefaultLister()))

lister.listFiles('Downloads')