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這是我的第一個問題，我是一個總noob。在Java中工作了3個月，所以要溫和。我正在做一個在線註冊跟蹤婚禮和用戶。用戶可以有多個婚禮，反之亦然。它本質上是多對多的關係，但我嘗試了另一條路線：將用戶和婚禮類別與陣列列表相互嵌入。像這樣。將一個帶有空ArrayList的對象添加到我的存儲庫中
package com.theironyard.Entities;
import javax.persistence.*;
import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;
/**
* Created by macbookair on 12/8/15.
*/
@Entity
@Table(name = "weddings")
public class Wedding {
public ArrayList <User> guests;
// ArrayList here is to store users associated with Wedding.
@Id
@GeneratedValue
@Column(nullable = false)
public int id;
@Column(nullable = false)
public String weddingName;
@Column(nullable = false)
public String location;
@Column(nullable = false)
public String date;
public Wedding(){
}
public Wedding(String date, String location, String weddingName, int id) {
this.date = date;
this.location = location;
this.weddingName = weddingName;
this.id = id;
}
public Wedding(ArrayList guests) {
this.guests = guests;
}
public int getId() {
return id;
}
public ArrayList getGuests() {
return guests;
}
public void setGuests(ArrayList guests) {
this.guests = guests;
}
public void setId(int id) {
this.id = id;
}
public String getWeddingName() {
return weddingName;
}
public void setWeddingName(String weddingName) {
this.weddingName = weddingName;
}
public String getLocation() {
return location;
}
public void setLocation(String location) {
this.location = location;
}
public String getDate() {
return date;
}
public void setDate(String date) {
this.date = date;
}
}
和用戶類
@Entity
@Table (name = "users")
public class User {
public ArrayList<Wedding>userWeddings;
// Array List to store weddings associated with user.
@Id
@GeneratedValue
@Column(nullable = false)
public int id;
@Column(nullable = false)
public String username;
@Column(nullable = false)
public String phone;
@Column(nullable = false)
public String email;
@Column(nullable = false)
public String password;
public User() {
}
public User(ArrayList<Wedding> userWeddings) {
this.userWeddings = userWeddings;
}
public User(int id, String username, String phone, String email, String password) {
this.id = id;
this.username = username;
this.phone = phone;
this.email = email;
this.password = password;
}
public ArrayList<Wedding> getUserWeddings() {
return userWeddings;
}
public void setUserWeddings(ArrayList<Wedding> userWeddings) {
this.userWeddings = userWeddings;
}
public int getId() {
return id;
}
public String getUsername() {
return username;
}
public String getPhone() {
return phone;
}
public String getEmail() {
return email;
}
public String getPassword() {
return password;
}
}
然後我運行一個「/創建婚禮」的路線，其增加了婚禮和用戶海誓山盟的ArrayList中。我們的目標是，以後大家可以參考一下誰去可以看到婚禮，一個人他們正在與像這樣相關聯的所有婚禮：
@RequestMapping(path = "/create-wedding", method = RequestMethod.POST)
public Wedding createWedding(@RequestBody Wedding wedding, HttpSession session) throws Exception {
weddings.save(wedding);
User user = users.findOneByEmail((String) session.getAttribute("email"));
if (user == null) {
throw new Exception("User does not exist");
}
wedding.guests.add(user);
//將用戶添加到「客人」數組列表婚禮課。
Invite invite = new Invite();
invite.isAdmin= true;
invite.wedding= wedding;
invite.email = user.email;
user.userWeddings.add(wedding); // adding user to List in "User class"
return wedding;
}
但我得到一個空指針異常，因爲數組列表爲空。我的印象是，如果我沒有明確地說他們是不可空的，那麼默認情況下他們會是。我考慮添加（nullable = true）註釋，但我似乎無法做到這一點，而不必將其作爲數據庫中的列。
基本上，我無法添加用戶/婚禮對象，因爲嵌入式ArrayLists爲空。即使解決這個問題，是否有一種更簡單的方法可以允許多個用戶訪問每個婚禮，反之亦然。