private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
        // TODO add your handling code here:
        string idStr=id.getText().trim();
        string nameStr=name.getText().trim();
        string sex=null;
        
        if(boy.isSelected())
        {
            sex="男";
        }
        else
        {
            sex="女";
        }
        if(idStr.length()>4&&nameStr.length()>1&&sex!=null)
        {
            message.setText("学生登记("+nameStr+")成功！");
            freeData();
        }
        else
        {
            message.setText("登记失败，请重新输入数据！");
        }
    }
  private void freeData()
  {
      id.setText("");
      name.setText("");
      boy.setSelected(true);
      girl.setSelected(false);
  }

前三行提示找不到符号，没有这个类，应该怎么办呢？

String  s大写

需要class