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Q1
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public class SetByInheritance extends Vector implements Set {

   public boolean isEmptySet() {   // 5 marks
      return isEmpty();
   }

   public boolean addItem(Object item) {  // 9 marks
      if (indexOf(item) == -1) {
         add(item);
         return true;
      } else {
         return false;
      }
   }

   public boolean removeItem(Object item) {  // 9 marks
      int index = indexOf(item);
      if (indexOf(item) == -1) {
         return false;
      } else {
         remove(index);
         return true;
      }
   }

      // if item is contained in Set, remove it and return true
      // otherwise return false
   public boolean containsItem(Object item) {  // 7 marks
      return (indexOf(item) != -1);
   }

   public int cardinality() {  // 5 marks
      return size();
   }
}

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Q2
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a) output:    2.5 marks each

7
false
true
false
false
true
false
43
true
false
true
false

b) Is the way equality testing has been implemented for classes A and B 
correct or not? If your answer is ``no'', then please explain your answer:

Yes, it is correct.  [5 marks]

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Q3 [6 marks each]
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(a) What is the main advantage of inheritance over composition?

substitutability (sub-type property)

(b) What are advantages of composition over inheritance?

simple, makes all methods explicit, everything is specified locally
easy to change underlying data-structure
[ even dynamically ]

(c) What is the difference between an abstract class and an interface?

an abstract class may supply some code and data members in addition
to method signatures; a subclass can only directly extend from one 
abstract class, but implement as many interfaces as you like


(d) Why would it not make sense for a method in Java to be declared both {\tt abstract} and {\tt final}?

abstract methods must be implemented in a non-abstract subclass, but
you could not do that if the method was declared final as well.

(e) What is the difference between the $==$ operator and the {\tt equals} method?

== tests for object identity (two references to exactly the same memory
location), whereas equals implements a notion of semantically equivalent
objects