What are the negative aspects of Java class Stack inheriting from Vector?

Question

By extending class Vector, Java’s designers were able to create class Stack quickly. What are the negative aspects of this use of inheritance, particularly for class Stack?<

Answer 1

Effective Java 2nd Edition, Item 16: Favor composition over inheritance:

Inheritance is appropriate only in circumstances where the subclass really is a subtype of the superclass. In other words, a class B should only extend a class A only if an "is-a" relationship exists between the two classes. If you are tempted to have a class B extend a class A, ask yourself this question: Is every B really an A? If you cannot truthfully answer yes to this question, B should not extend A. If the answer is no, it is often the case that B should contain a private instance of A and expose a smaller and simpler API; A is not an essential part of B, merely a detail of its implementation.

There are a number of obvious violations of this principle in the Java platform libraries. For example, a stack is not a vector, so Stack should not extend Vector. Similarly, a property list is not a hash table, so Properties should not extend Hashtable. In both cases, composition would have been preferrable.

The book goes in greater detail, and combined with Item 17: Design and document for inheritance or else prohibit it, advises against overuse and abuse of inheritance in your design.

Here's a simple example that shows the problem of Stack allowing un-Stack-like behavior:

    Stack stack = new Stack();
    stack.push("1");
    stack.push("2");
    stack.push("3");
    stack.insertElementAt("squeeze me in!", 1);
    while (!stack.isEmpty()) {
        System.out.println(stack.pop());
    }
    // prints "3", "2", "squeeze me in!", "1"

This is a gross violation of the stack abstract data type.

See also

• Wikipedia/Stack (data structure)

  In computer science, a stack is a last in, first out (LIFO) abstract data type and data structure.