若要从数据库中检索的ResultSet不存在，该怎么办.如何在JDBC中使命令发出JOptionPane消息

Question

我想知道如果单击搜索按钮后ID不存在，我在哪里输入命令以显示错误。。

else if(e.getSource() == btnSearch) {
  ResultSet rs = null;           
  rs = sD.getSummonID(txtSummonID.getText());
  Summon s = null;
  try {
    while(rs.next()) {
      s = new Summon(rs.getInt("id"), rs.getString("plateNum"), 
          rs.getDouble("amount"), rs.getString("recDate"), 
          rs.getString("location"), rs.getString("officer")); 
    }
    txtcarLicenceNo.setText(s.getCarLicenceNo());
    txtAmount.setText(Double.toString(s.getAmount()));
    txtLocation.setText(s.getLocation());
    txtOfficer.setText(s.getOfficerInCharge());                       
  } catch (SQLException ex) {
    System.err.println("SQL Exception: " + ex);
  }
}

另一类看起来像这个

public ResultSet getSummonID(String summonID) {
  getConnection();
  try {
    stmt = conn.createStatement();
    rs = stmt.executeQuery("SELECT * FROM record WHERE id = " + summonID);                 
    return rs;
  } catch (SQLException e) {
    System.err.println("SQL Exception: " + e);
    return rs;
  }
}

Answer 1

rs = sD.getSummonID(txtSummonID.getText());
if(condition to check that your id is null) {
   //display pop up
   JOptionPane.showMessageDialog(frame, "Id does not exists");
}else {
  // proceed further
  Summon s = null;
    try {
        while(rs.next())
        {s = new Summon(rs.getInt("id"), rs.getString("plateNum"), 
                rs.getDouble("amount"), rs.getString("recDate"), 
                rs.getString("location"), rs.getString("officer")); 
        }
          txtcarLicenceNo.setText(s.getCarLicenceNo());
          txtAmount.setText(Double.toString(s.getAmount()));
          txtLocation.setText(s.getLocation());
          txtOfficer.setText(s.getOfficerInCharge());                       
        }            
    catch (SQLException ex) 
    {
        System.err.println("SQL Exception: " + ex);
    }
}

Answer 2

使用当前代码，当由于Summon对象s初始化为null而没有结果提取时，您将获得NullPointerException：

    Summon s = null;

并且当没有结果时

 while(rs.next())

永远不会迭代来设置s对象。为了避免空指针异常，您可以引入这里提到的空检查：

while(rs.next())
            {s = new Summon(rs.getInt("id"), rs.getString("plateNum"), 
                    rs.getDouble("amount"), rs.getString("recDate"), 
                    rs.getString("location"), rs.getString("officer")); 
            }
              if(s != null) {
              txtcarLicenceNo.setText(s.getCarLicenceNo());
              txtAmount.setText(Double.toString(s.getAmount()));
              txtLocation.setText(s.getLocation());
              txtOfficer.setText(s.getOfficerInCharge());                       
              } else {
                   // display JOptionPane
                  JOptionPane.showMessageDialog(frame, "Your error/info message");
              }
            }