从会话添加数组列表但我得到了错误

Question

     - -package example;

           import com.opensymphony.xwork2.ActionContext;
           import com.opensymphony.xwork2.ActionSupport;
           import example.Contact;
           import java.util.ArrayList;
           import java.util.Map;

           /**
            *
            * @author LENOVO
            */
           public class ContactAction extends ActionSupport {

               private String msg;
               private Contact contact;

               public Contact getContact() {
                   return contact;
               }

               public void setContact(Contact contact) {
                   this.contact = contact;
               }

               public String addContact() throws Exception {
                   ArrayList<Contact> contacts;
                   Map session = ActionContext.getContext().getSession();
                   contacts = (ArrayList) session.get("contacts");
                   if (contacts == null) {
                       contacts = new ArrayList<>();
                       contacts.add(contact);
                       session.put("contacts", contact);
                   } else {

                       contacts.add(contact);
                       session.put("contacts", contacts);
                   }

                   return SUCCESS;

               }


 }