public function getStopwords()
    {
        if(!empty($this->stopWords)) {
            return $this->stopWords;
        }
                
        foreach($this->getFilePaths() as $filePath)
        {
            $content = $this->getFileContent($filePath);
            $doc = new TokensDocument((new GeneralTokenizer())
                    ->tokenize($content) );
            $doc->applyTransformation(new LowerCaseFilter())
                ->applyTransformation(new PossessiveNounFilter())
                ->applyTransformation(new PunctuationFilter())
                ->applyTransformation(new CharFilter());
            
            if($this->mode === self::MODE_FREQ) {
                $this->computeUsingFreqDist($doc->getDocumentData());
            }
            
        }
        arsort($this->stopWords);
        return $this->stopWords;                        
    }