docs: private Map<String, Double> getTF(String content) {
  		Map<String,Double> tfMap=new HashMap<>();
  		if(content==null || content.equals(""))
  			return tfMap; 
  		
  		JiebaSegmenter segmenter = new JiebaSegmenter();
  		List<String> segments=segmenter.sentenceProcess(content);
  		Map<String,Integer> freqMap=new HashMap<>();
  		
  		int wordSum=0;
  		for(String segment:segments) {
  			//停用词不予考虑，单字词不予考虑
  			if(!stopWordsSet.contains(segment) && segment.length()>1) {
  				wordSum++;
  				if(freqMap.containsKey(segment)) {
  					freqMap.put(segment,freqMap.get(segment)+1);
  				}else {
  					freqMap.put(segment, 1);
  				}
  			}
  		}
  		
  		// 计算double型的tf值
  		for(String word:freqMap.keySet()) {
  			tfMap.put(word,freqMap.get(word)*0.1/wordSum);
  		}
  		
  		return tfMap; 
  	}