error: Entities and Pojos must have a usable public constructor - Java

Question

每当我尝试编译应用程序时，我都会收到此错误

error: Entities and Pojos must have a usable public constructor. You can have an empty constructor or a constructor whose parameters match the fields (by name and type). - java.util.List

SystemMessagesEntity.java :

    @Entity(tableName = "system_messages")
    @TypeConverters(ReplyMessages.class)
    public class SystemMessageEntity {
        @PrimaryKey
        @NonNull
        public String messageId;
        public String title;
        public String body;
        public String color;
        public Integer date;
        public String icon;
        public String ad;
        public Integer isRead;
        public String ip;
        String id; //user_id
        @Embedded(prefix = "reply_")
        private List<ReplyMessage> reply_message;
    
        @Ignore
        public SystemMessageEntity() {
        }
    
        public SystemMessageEntity(String id, @NonNull String messageId, String title, String body, String color, Integer date, String icon, String ad, Integer isRead, String ip, List<ReplyMessage> reply_message) {
            this.id = id;
            this.messageId = messageId;
            this.title = title;
            this.body = body;
            this.color = color;
            this.date = date;
            this.icon = icon;
            this.ad = ad;
            this.isRead = isRead;
            this.ip = ip;
            this.reply_message = reply_message;
        }
    //getters and setters
}

ReplyMessages.java :

@TypeConverters(ReplyMessages.class)

public class 回复留言 {

private String body = "";
private String userId = "";
private Integer date = 0;
private String id = "";
private String messageId = "";


@Ignore
public ReplyMessage() {
}

public ReplyMessage(String body, String userId, Integer date, String id, String messageId) {
    this.body = body;
    this.date = date;
    this.id = id;
    this.messageId = messageId;
    this.userId = userId;
}

这是 TypeConverter :

public class ReplyMessages {

    @TypeConverter
    public static String ListToJson(List<ReplyMessage> replyMessages) {
        if(replyMessages == null) return null;
        Type type = new TypeToken<List<ReplyMessage>>() {}.getType();
        String json = new Gson().toJson(replyMessages, type);
        Log.i("JSON", "toJson: " + json);
        return replyMessages.isEmpty() ? null : json;
    }

    @TypeConverter
    public static List<ReplyMessage> JsonToList(String json) {
        Gson gson = new Gson();
        Type type = new TypeToken<List<ReplyMessage>>() {}.getType();
        List<ReplyMessage> replyMessages = gson.fromJson(json, type);
        return replyMessages;
    }
}

我想使用 Retrofit 从 API 获取 JSON 数据并将其存储在数据库中，JSON 数据有一个对象数组 (MessageReply)在对象的父数组中 (SystemMessagesEntity).

我已经尝试了所有方法并搜索了 Whosebug 中的几乎每个问题，但我什么也没找到。

编辑：当我单独使用 SystemMessageEntity 实体时，数据库运行良好。但是当我添加 ReplyMessage 实体时，问题出现了。

编辑 2：问题已通过删除 ReplyMessage 的 List<> 得到解决。虽然我正在使用转换器，但我不知道为什么列表不起作用。

Answer 1

Android Room 需要一个空构造函数，因此从空构造函数中删除 @Ignore 注解并将其放在带参数的构造函数上。

 public SystemMessageEntity() {}

 @Ignore
 public SystemMessageEntity(String id, @NonNull String messageId, ...) {
     // ... initializing the params
 }

Answer 2

我已经解决了问题。好吧，似乎我不能将 @Embedded 与 @TypeConverter 一起使用，主要是因为转换器 returns 是一个字符串，它是原始的而不是 POJO class。无论如何，我删除了@Embedded 注释并且它工作正常。

error: Entities and Pojos must have a usable public constructor - Java

error: Entities and Pojos must have a usable public constructor - Java

java

android

pojo

android-room