Guard an Empty Array Queue

ShiftQueue can already accept values and print itself, but nothing stops it reading a slot it does not own. Three missing methods have to agree.

Requirements:
1. isEmpty() reports whether the queue holds no values.
2. peek() returns -1 when empty, otherwise the front value, removing nothing.
3. dequeue() also returns -1 when empty, otherwise saves items[0], shifts the later values one slot left, decreases size and returns the saved value.
4. peek() and dequeue() must both decide the empty case by calling isEmpty().

Do NOT write a main method.