“Type of the parameter must be a class annotated w

问题:

I am using Room architecture component for persistence. I have created generic DAO interface to avoid boilerplate code. Room Pro Tips

But my code doesn't compile saying "Error:(21, 19) error: Type of the parameter must be a class annotated with @Entity or a collection/array of it." for the Generic class T.

interface BaseDao<T> {

@Insert(onConflict = OnConflictStrategy.REPLACE)
void insert(T... entity);

@Update
void update(T entity);

@Delete
void delete(T entity);
}

@Dao
public abstract class ReasonDao implements BaseDao<ReasonDao> {

   @Query("SELECT * from Reason")
   abstract public List<Reason> getReasons();

}

Is there anything I am missing here. It works like this here

回答1:

I had initially followed the method used in Kotlin, but that gives the error in Java code. Two quick changes fixed it for me

• Change BaseDao to Abstract class
• Added @Dao annotation to the BaseDao

Please find the code below and now it runs properly

@Dao
abstract class BaseDao<T> {

   @Insert(onConflict = OnConflictStrategy.REPLACE)
   abstract void insert(T entity);

   @Update
   abstract void update(T entity);

   @Delete
   abstract void delete(T entity);
 }

 @Dao
 public abstract class ReasonDao extends BaseDao<Reason>{

    @Query("SELECT * from Reason")
    abstract public List<Reason> getReasons();

  }

回答2:

I believe is that you have missed to give Entity annotation to T class. For example Reason class should have @Entity and give it to ReasonDao class. Like:

@Dao public abstract class ReasonDao extends BaseDao<Reason>{}

回答3:

In my case I tried to save to DB non-Entity objects. Then replaced with Entity class (contains @Entity(tableName = "your_table", indices = [Index("your_key")])).

回答4:

The reason is that you specified ReasonDao type as generic parameter instead of Reason.

Original code:

@Dao
public abstract class ReasonDao implements BaseDao<ReasonDao> {

   ...

}

Correct code:

@Dao
public abstract class ReasonDao implements BaseDao<Reason> {

   ...

}

where Reason is the type marked with @Entity annotation.

By the way, this is fixed in the accepted answer, but is not mentioned in the changelist :)