java - jdbc 中的结果集返回 true

Question

我编写了以下代码，即使没有结果，结果集(此处标记为 rs3)也返回 true。我想检查是否没有结果，然后用户必须再次输入正确的手机制造商。Don不知道我哪里错了？

 BufferedReader r2 = new BufferedReader(new InputStreamReader(System.in)); 
 String phone_manufacturer="";
 boolean value1=true;
 while (value1) {
    System.out.println("\nPlease select your choice of phone manufacturer " );
    String line = r2.readLine();
    if (line.matches("[A-Za-z ]+$")) {
        phone_manufacturer = line;
        final String sql3 = "SELECT * from phone_model WHERE phone_type = '"+phone_type_choice+"' and manufacturer ='"+phone_manufacturer+"'";
        st3 = connection.createStatement();
        rs3= st3.executeQuery(sql3);
        if(rs3!=null){
              System.out.println("Model"+"      "+"Manufacturer"+""+"Phone_type");
             while(rs3.next()){
                String modell  = rs3.getString("Model");
                String manufacturer = rs3.getString("Manufacturer");
                String phone_type = rs3.getString("Phone_type");
          System.out.format("%-25s%-20s%-10s\n",modell,manufacturer,phone_type);
            }
         }

         else
          {
       System.out.println("The manufacturer isn't avaiable for the phone type choosen.Please try again");
           value1=true;
           continue;
             }

     value1=false;
        }else
           {
           System.out.println("\nPlease enter correct manufacturer name " );
                            value1=true;
                            continue;

                          }
                          break;
                          }

Answer 1

用途:

if (!rs3.next() ) {
    System.out.println("no data");
}

最初 ResultSet 的光标指向第一行之前，如果第一次调用 next() 返回 false 则 ResultSet 中没有数据。

如果您正在使用新返回的 ResultSet，其光标指向第一行之前，则更简单的检查方法是调用 isBeforeFirst()。

if (!rs3.isBeforeFirst() ) {    
 System.out.println("No data"); 
} 

最好使用准备好的语句来避免sql注入(inject):

 PreparedStatement updateemp = connnection.prepareStatement
      ("SELECT * from phone_model WHERE phone_type =? and manufacturer=?");
      updateemp.setString(1,phone_type_choice);
      updateemp.setString(2, phone_manufacturer);

How To use prepared statement.