Allowing non-POJO Firestore classes to be subclassed for mocking

google-cloud-firestore/src/main/java/com/google/cloud/firestore/CollectionReference.java

@@ -26,9 +26,13 @@
 
 /**
  * A CollectionReference can be used for adding documents, getting document references, and querying
- * for documents (using the methods inherited from Query)
+ * for documents (using the methods inherited from Query).
+ *
+ * <p><b>Subclassing Note</b>: Firestore classes are not meant to be subclassed except for use in
+ * test mocks. Subclassing is not supported in production code and new SDK releases may break code
+ * that does so.
  */
-public final class CollectionReference extends Query {
+public class CollectionReference extends Query {
 
   /**
    * Creates a CollectionReference from a complete collection path.

google-cloud-firestore/src/main/java/com/google/cloud/firestore/DocumentReference.java

@@ -36,8 +36,12 @@
  * write, read, or listen to the location. There may or may not exist a document at the referenced
  * location. A DocumentReference can also be used to create a CollectionReference to a
  * subcollection.
+ *
+ * <p><b>Subclassing Note</b>: Firestore classes are not meant to be subclassed except for use in
+ * test mocks. Subclassing is not supported in production code and new SDK releases may break code
+ * that does so.
  */
-public final class DocumentReference {
+public class DocumentReference {
 
   private final ResourcePath path;
   private final FirestoreImpl firestore;

google-cloud-firestore/src/main/java/com/google/cloud/firestore/DocumentSnapshot.java

@@ -41,6 +41,10 @@
  * <p>If the DocumentSnapshot points to a non-existing document, getData() and its corresponding
  * methods will return null. You can always explicitly check for a document's existence by calling
  * {@link #exists()}.
+ *
+ * <p><b>Subclassing Note</b>: Firestore classes are not meant to be subclassed except for use in
+ * test mocks. Subclassing is not supported in production code and new SDK releases may break code
+ * that does so.
  */
 public class DocumentSnapshot {
 

google-cloud-firestore/src/main/java/com/google/cloud/firestore/QueryDocumentSnapshot.java

@@ -32,8 +32,12 @@
  * <p>QueryDocumentSnapshot offers the same API surface as {@link DocumentSnapshot}. Since query
  * results contain only existing documents, the {@link #exists()} method will always return true and
  * {@code getData()} will never be null.
+ *
+ * <p><b>Subclassing Note</b>: Firestore classes are not meant to be subclassed except for use in
+ * test mocks. Subclassing is not supported in production code and new SDK releases may break code
+ * that does so.
  */
-public final class QueryDocumentSnapshot extends DocumentSnapshot {
+public class QueryDocumentSnapshot extends DocumentSnapshot {
   private QueryDocumentSnapshot(
       FirestoreImpl firestore,
       DocumentReference docRef,