JAVA中空值String拼接的坑！！！

看下面代码

String result = "";
		PreparedStatement  ps = null;
		try {
			ps = conn.prepareStatement("SELECT * FROM student");
			ResultSet rs = ps.executeQuery();
			while(rs.next()){
				int id = rs.getInt("id");
				String name = rs.getString("name");
				int age = rs.getInt("age");
				String cla = rs.getString("class");
				result += ""+ id +""+ name +""+ age +" "  +cla+ " ";
				
			}
			
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}finally {
			// 5. 关闭查询
			try {
				if (null != ps) {
					ps.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}
		return result;
		

一个空值null的字符串加上了一个值之后空值则变成了‘null’的一个字符串了！！！