Optional instance with no contained reference.Set are disjoint.Set whose only element is the contained instance if it is present; an empty
immutable Set otherwise.true if object is an Optional instance, and either the contained references are
equal to each other or both are absent.ResponseField.Type.BOOLEAN.ResponseField.Type.CUSTOM_LIST.ResponseField.Type.CUSTOMResponseField.Type.DOUBLE.ResponseField.Type.ENUM.ResponseField.Type.FRAGMENT.ResponseField.Type.INLINE_FRAGMENT.ResponseField.Type.INT.ResponseField.Type.LONG.ResponseField.Type.OBJECT.ResponseField.Type.OBJECT_LIST.ResponseField.Type.SCALAR_LIST.ResponseField.Type.STRING.nullableReference is non-null, returns an Optional instance containing that reference; otherwise
returns Optional.absent().true if this holder contains a (non-null) instance.Optional instance containing the given non-null reference.defaultValue otherwise.Optional if it has a value present; secondChoice otherwise.null otherwise.D.toString() on its argument.Function; otherwise, Optional.absent()
is returned.D with another class.