public static Map<String, Double> getWordIDFMap(List<String> wordList) {
		
		Map<String, Double> wordIDFMap = new HashMap<String ,Double>();
		
		StringBuilder wordListBuf = new StringBuilder();
		wordListBuf.append("(");
		for(String word: wordList) {
			wordListBuf.append("'").append(word).append("'").append(",");
		}
		wordListBuf.deleteCharAt(wordListBuf.length() - 1);
		wordListBuf.append(")");
		
		String sql = "select word, idf from inverted_index where word in " + wordListBuf.toString();
		System.out.println(sql);
		
		try{
			conn = DBConnection.getConnectionInstance();
			
			
			PreparedStatement preparedStatement = conn.prepareStatement(sql);
			
			ResultSet rs = preparedStatement.executeQuery();
			while(rs.next()) {
				
				String word = rs.getString(1);
				Double idf = rs.getDouble(2);
			
				wordIDFMap.put(word, idf);
			}
		}
		catch(Exception e) {
			
			System.out.println("getWordIDFMap failure !");
			e.printStackTrace();
		}
		
		return wordIDFMap;
	}