Invalid state means invalid state. Suppose (going with my bounded stack example from another reply to you) you initialize the stack with: a_bounded_stack = n

Invalid state means invalid state. Suppose (going with my bounded stack example from another reply to you) you initialize the stack with: a_bounded_stack = new BoundedStack(100); There are various actions applied, correctly, through the interface (push, pop, and peek). Somewhere, you decide to manually alter the size of the stack but not through its interface (resize) a_bounded_stack.limit = 50; But you forget (someone came by with cookies) to actually shrink the backing array and also to change "top". So now you end up with this state: a_bounded_stack limit = 50 array.size = 100 top = 60 Oops, top is not allowed to be larger than limit, and array.size is supposed to be equal to limit. That is an invalid state, people can still use the public interface (push, pop, peek), but who knows what they'll get back.