src/main/java/com/googlecode/objectify/ObjectifyFactory.java
public <T> KeyRange<T> allocateIds(final Class<T> clazz, final int num) {
		final String kind = Key.getKind(clazz);
		final IncompleteKey incompleteKey = datastore().newKeyFactory().setKind(kind).newKey();

		return allocate(incompleteKey, num);
	}
public <T> KeyRange<T> allocateIds(final Class<T> clazz, final int num) {
		final String kind = Key.getKind(clazz);
		final IncompleteKey incompleteKey = datastore().newKeyFactory().setKind(kind).newKey();

		return allocate(incompleteKey, num);
	}
<p>Preallocate multiple unique ids within the namespace of the
specified entity class.  These ids can be used in concert with the normal
automatic allocation of ids when save()ing entities with null Long id fields.</p>

<p>The {@code KeyRange<?>} class is deprecated; when using this method,
treat the return value as {@code List<Key<T>>}.</p>

@param clazz must be a registered entity class with a Long or long id field.
@param num must be >= 1 and small enough we can fit a set of keys in RAM.