Best practice for using generics with Firebase snapshot.getValue()

TL;DR: How to correctly use a generic class with Firebase DataSnapshot.getValue()?

Use case: I want to implement a single generic remote datasource class for all my entities (a bunch of them) using Firebase. When listening to data change, I want to get the values from datasnapshot as an object of type E (its type determined elsewhere) but I don’t know if it is doable with Firebase query, like the following:

public class GenRemoteDataSource<E extends SomeClass>
{
   //...
   public void onDataChange(DataSnapshot dataSnapshot)
   {
        E item = (E) dataSnapshot.getValue(); // <- unchecked cast and it doesn't work
        items.add(item);
   }
}

JavaScript
​x
 
public class GenRemoteDataSource<E extends SomeClass>{   //...   public void onDataChange(DataSnapshot dataSnapshot)   {        E item = (E) dataSnapshot.getValue(); // <- unchecked cast and it doesn't work        items.add(item);   }}​

For example, I have a Foo class that extends SomeClass, and the implementation of this GenRemoteDataSource with a class of Foo would be:

public class Foo extends SomeClass{}

public class FooRemoteDataSource extends GenRemoteDataSource<Foo>
{
   //...
}

JavaScript
 
public class Foo extends SomeClass{}​public class FooRemoteDataSource extends GenRemoteDataSource<Foo>{   //...}​

but Firebase throws a runtime error because instead of casting the getValue() as Foo, it would try to cast value as a upper bound SomeClass instead. I’m baffled as to why this happens:

Java.lang.ClassCastException: java.util.HashMap cannot be cast to com.example.app.SomeClass

JavaScript
 
Java.lang.ClassCastException: java.util.HashMap cannot be cast to com.example.app.SomeClass​

please advise on how should I do this with Type-safety (No unchecked cast) . Thank you.

EDIT Stuff turned out to be irrelevant down below, see GenericTypeIndicator

EDIT I’ve also tried (blindly and heck worth a try) the GenericTypeIndicator,

GenericTypeIndicator<E> mTypeIndicator = new GenericTypeIndicator<>();
E item = dataSnapshot.getValue(mTypeIndicator);

JavaScript
 
GenericTypeIndicator<E> mTypeIndicator = new GenericTypeIndicator<>();E item = dataSnapshot.getValue(mTypeIndicator);​

but it instead spits the following runtime error.

com.google.firebase.database.DatabaseException: Not a direct subclass of GenericTypeIndicator: class java.lang.Object

JavaScript
 
com.google.firebase.database.DatabaseException: Not a direct subclass of GenericTypeIndicator: class java.lang.Object​

Answer

TL;DR my solution, (introduces more dependency than semantically necessary ?)

public class GenRemoteDataSource<E extends SomeClass>
{
   //...
   private final Class<E> clazz;

   GenRemoteDataSource(Class<E> clazz)
   {
      this.clazz = clazz;
   }

   //...
   public void onDataChange(DataSnapshot dataSnapshot)
   {
        E item = dataSnapshot.getValue(clazz); // <- now is type safe.
        items.add(item);
   }
}

JavaScript
 
public class GenRemoteDataSource<E extends SomeClass>{   //...   private final Class<E> clazz;​   GenRemoteDataSource(Class<E> clazz)   {      this.clazz = clazz;   }​   //...   public void onDataChange(DataSnapshot dataSnapshot)   {        E item = dataSnapshot.getValue(clazz); // <- now is type safe.        items.add(item);   }}​

I ended up just providing the Class definition to the getValue() via a constructor injection. This is done by passing a Class<T> into the generic class’s constructor and sets it for Firebase’s getValue() to use as a instance parameter. Please comment if there are better solution, because to me this seems redundant. Thank You!

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Answer