错误指出while(res.next()){这一行空指针异常，请问原因出在哪

Question

public class sqlproject {
private static Connection con = null;
private static ResultSet res = null;
public static void getLink(){
try {
Class.forName("com.mysql.cj.jdbc.Driver");
con = DriverManager.getConnection("jdbc:mysql://localhost:3306/first?serverTimezone=UTC","root","123");
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
}
public static void getResult(){
try {
Statement sta = con.createStatement();
res = sta.executeQuery("select *from first;");
if(res!=null){
System.out.println("keyi1");
}else System.out.println("nmo");
} catch (SQLException e) {
e.printStackTrace();
}

}
public static List  backResult()  {
    List<sqlgather> list = new ArrayList<>();
    try{
    while(res.next()){
        String name = res.getString("name");
        int id = res.getInt("id");
        String sex = res.getString("sex");
        int age = res.getInt("age");
        list.add(new sqlgather(name,id,sex,age));
    }
}catch(SQLException e){
        e.printStackTrace();
    } return list;
}

}

Answer 1

你的 main 方法是怎么写的，从代码逻辑来看，调用顺序应该是：

getLink()；
getResult();
backResult()

而且 backResult() 应该优化一下，res.next 之前先判断一下 res !=null 。