src/main/java/com/googlecode/objectify/ObjectifyFactory.java
public <T> KeyRange<T> allocateIds(final Object parentKeyOrEntity, final Class<T> clazz, final int num) {
		final Key<?> parent = keys().anythingToKey(parentKeyOrEntity);
		final String kind = Key.getKind(clazz);

		final IncompleteKey incompleteKey = com.google.cloud.datastore.Key.newBuilder(parent.getRaw(), kind).build();

		return allocate(incompleteKey, num);
	}
public <T> KeyRange<T> allocateIds(final Object parentKeyOrEntity, final Class<T> clazz, final int num) {
		final Key<?> parent = keys().anythingToKey(parentKeyOrEntity);
		final String kind = Key.getKind(clazz);

		final IncompleteKey incompleteKey = com.google.cloud.datastore.Key.newBuilder(parent.getRaw(), kind).build();

		return allocate(incompleteKey, num);
	}
Preallocate a contiguous range of unique ids within the namespace of the
specified entity class and the parent key.  These ids can be used in concert with the normal
automatic allocation of ids when put()ing entities with null Long id fields.

@param parentKeyOrEntity must be a legitimate parent for the class type.  It need not
point to an existent entity, but it must be the correct type for clazz.
@param clazz must be a registered entity class with a Long or long id field, and
a parent key of the correct type.
@param num must be >= 1 and <= 1 billion