Use inheritance as an implementation technique

when you are going to use inheritance as an implementation technique, you can achieve same goal in a safer way by making one class an attribute or associate of other class. In this way, one object could selectively invoke desired functions of another class, by using delegation rather which applying inheritance. 

A safer implementation of Stack will delegate to List class as shown in Figure. Every instance of Stack includes a private instance of List. Stack: push operation delegates to list by calling its last and add operations to add the element at the end of list, and the pop operation has a same implementation using last and remove operations. The ability to corrupt stack by adding or removing arbitrary elements is hidden from client of the Stack class.