处理异常返回值，代码调优

src/main/java/com/example/springdemo/dao/impl/UsersDaoImpl.java

@@ -19,6 +19,7 @@ public class UsersDaoImpl implements UsersDao {
     private PreparedStatement preparedStatement = null;
     private ResultSet resultSet = null;
     DataBaseUtil dataBaseUtil = new DataBaseUtil();
+
     //新建用户
     @Override
     public int insert(@NotNull Users users) {
@@ -46,7 +47,7 @@ public class UsersDaoImpl implements UsersDao {
     @Override
     public int delete(@NotNull Users user) {
         String delete_sql = "delete from Users where id=?";
-        CommonDao<Users> dao = new CommonDao<>();
+        CommonDao dao = new CommonDao();
         return dao.delete(connection, preparedStatement, resultSet,
                 delete_sql, user.getId());
     }
@@ -121,8 +122,13 @@ public class UsersDaoImpl implements UsersDao {
             dataBaseUtil.close(connection, preparedStatement, resultSet);
         }
 
+        if (user == null) {
+            throw new RuntimeException("用户不存在");
+        } else {
             return user;
         }
+    }
+
     private void SetAttribute(@NotNull Users user, @NotNull ResultSet rs) throws SQLException {
         user.setId(rs.getLong("id"));
         user.setName(rs.getString("name"));