为啥会出现Null pointer access: The variable lists can only be null at this location错误
String sql = "select flow_id, type, id_card, exam_card, student_name, location, grade from examstudent";
PreparedStatement ps = null;
ResultSet rs = null;
List<ExamStudent> lists = null;
try {
ps = connection.prepareStatement(sql);
rs = ps.executeQuery();
while(rs.next()) {
ExamStudent examstudent = new ExamStudent();
int i = 1;
examstudent.setFlow_id(rs.getInt(i++));
examstudent.setType(rs.getInt(i++));
examstudent.setId_card(rs.getString(i++));
examstudent.setExam_card(rs.getString(i++));
examstudent.setStudent_name(rs.getString(i++));
examstudent.setLocation(rs.getString(i++));
examstudent.setGrade(rs.getFloat(i++));
lists.add(examstudent); //Null pointer access: The variable lists can only be null at this location
}
} catch (SQLException e) {
e.printStackTrace();
}
System.out.println(lists.size());
return lists;