Regarding extension of interfaces Graph into subinterfaces MutableGraph

[amcp]

Hello,
I have an interface Graph and an interface MutableGraph which extends Graph.

I know am writing the implementation of this interface hierarchy,
but I do not know about the consequences of having my class implement the superinterface as opposed to, say, both the super and the sub.

If I implement the sub's interface, will I still be able to cast instances of the class as the superinterface?
Or, to do this, do I need to have the class implement both?

thank you

[Greg Belote]

For anything that implements the subinterface, it is considered an instanceof both the sub and super. So yeah, you can cast it as the super interface.

[C. Scott Ananian (TA)]

Be aware that you should be disciplined in your use of these dual interfaces.  A mutable graph "is a" immutable graph, which may surprise some users -- especially if you pass an "immutable" graph to some code and then start changing it (via it's mutable subclass).

This is a reasonable design decision, but be sure to carefully document the gotchas.