public static InvertedIndex getInvertedIndex(String word) {
		InvertedIndex invertedIndex = null;
		
		try{
			conn = DBConnection.getConnectionInstance();
			String sql = "select * from inverted_index where word = ?";
			
			PreparedStatement preparedStatement = conn.prepareStatement(sql);
			preparedStatement.setString(1, word);
			
			int cnt = 0;
			ResultSet rs = preparedStatement.executeQuery();
			invertedIndex = new InvertedIndex();
			while(rs.next()) {
				++cnt;
				
				invertedIndex.setInvertedId(rs.getInt(1));
				invertedIndex.setWord(rs.getString(2));
				invertedIndex.setNdocs(rs.getInt(3));
				invertedIndex.setDocListStr(rs.getString(4));
				invertedIndex.setIdf(rs.getDouble(5));
			}
			
			if(cnt == 0)
				return null;
			
		}
		catch(Exception e) {
			
			System.out.println("get failure !");
			e.printStackTrace();
		}
		
		return invertedIndex;
	}