java - 必须用@NonNull(复合主键)注释主键

Question

我的 Android 项目中有以下 Java 类。

@Entity
public class Daily {

    @PrimaryKey
    private Date dailyId;

    //Other non important attrs, getters, setters, etc.

}

@Entity(primaryKeys = {"dailyId", "dailyDetailId"})
public class DailyDetail {

    private Date dailyId; //which is the value of its unique parent.
    private Long dailyDetailId;

    //Other non important attrs, getters, setters, etc.

}

顺便说一句:我已经添加了类型转换器。

当我尝试构建项目时出现以下错误:

You must annotate primary keys with @NonNull. "dailyId" is nullable. SQLite considers this a bug and Room does not allow it.

然后，当我按照说明将 @NonNull 添加到 dailyid 时，它显示 必须初始化非空字段 (?)

我应该如何解决这个问题？我的想法是初始化两个主键，但是当我尝试将新对象插入数据库时​​，这应该是一个问题。

Answer 1

Then, when I follow the instructions and add @NonNull to dailyid, it says Not-null fields must be initialized (?)

如果你用@NonNull注释一个字段，你就是在告诉编译器“这个东西永远不会为空”。但是Java中未初始化对象的默认值是多少呢？这是正确的! 空。因此，如果您使用 @NonNull 注释字段，您必须初始化它以保证它不会从 null 开始。

How should I fix this?

初始化您的字段。立即声明或在类构造函数中。

@NonNull
@PrimaryKey
private Date dailyId = new Date(); // Now it's initialized and not null

或者

@Entity
public class Daily {

    @NonNull
    @PrimaryKey
    private Date dailyId;

    public Daily() {
        dailyId = new Date(); // Now it's initialized and not null
    }

    // ^- this AND / OR this -v

    // Note here that if using an argument in the constructor, it too must be
    // annotated as @NonNull to tell the compiler that you're setting the value
    // of your non-nullable field to something that won't itself be null
    public Daily(@NonNull Date initialDate) {
        dailyId = initialDate; // Now it's initialized and not null
    }
}

希望有帮助!