Assignment 9
Due 11/09/04 12:30 p.m.
(Must be done with your group)

1. [synthesis] The static and dynamic links for an activation record may both point to the same activation record or to different activation records.
   1. Give an example where the static and dynamic links of an activation record, A, both point to an activation record B.  Explain your example.
   2. Give an example when the static and dynamic links of an activation record, A, point to different activation records.  Explain your example.
2. Consider routines, A, An1, An2, and B.  Assume that An1 and An2 are immediately nested inside A, and A and B are immediately nested inside main. 
   1. [skill 16.2] Main calls A, which calls An1, which calls An2, which calls B.  Show (using ascii art) or describe what the stack looks like when B is called.
   2. [skill 16.3] Assuming static scoping, describe how An2 accesses a variable declared inside A.  Be clear about how you will traverse static and/or dynamic links to access the variable.
3. [synthesis] While C++ supports multiple inheritance of classes, Java does not. 
   1. Does multiple inheritance add complexity to the language?  Explain and give examples to support your position.
   2. Do you think that multiple inheritance is useful to have?  Explain giving examples to support your position.
4. [skill 17.3] Consider the following class hierarchy in which all methods are virtual (which is the default in Java):
   class A {
     void m() { print "A's m implementation"; }
   }
   class B extends A {
     void m() { print "B's m implementation"; }
     void n()  { print "B's n implementation"; }
   }
   class C extends B {
     void n() { print "C's n implementation"; }
   }
   i.e., C inherits from B and B inherits from A.  Also B overrides A's m method and C overrides B's n method. 
   What will be the output of the following code fragment.  Explain your answer.
   /* fragment 1 */
   A a = new B();
   a.m();
   
   /* fragment 2 */
   A a = new B();
   a.n();
    
5. [skills 17.1, 17.2] Come up with a real example where you use inheritance and dynamic dispatching.  Your example should be based on a real problem and not contrived.  Write the entire program in C++ or Java.  Your code should compile and run.
6. [skill 18.1] Explain the benefits of including interface types in Java.  Give one example to support your answer.
7. [skill 18.2] Section 5.1.5 of the Java language definition (titled "Narrowing Reference Conversions") gives the narrowing conversions that Java allows.  Pick three of these conversions and for each of them give a successful example of the conversion (I say "successful" because a narrowing conversion may be successful or it may fail with a run-time error).
8. [skill 18.2] Consider the following code fragment:
   interface I {
     void f();
   }
   class A implements I {
     void f() { print "A's f implementation"; }}
   }
   class B implements I {
     void f() {print "B's f implementation"; }}
   1. What are the subtyping relationships between A, B, and I.
   2. Explain why the following code is legal and what it will print.  Assume that b is a boolean variable declared and initialized elsewhere.
      {
        I i;
        if (b) {
          i = new A();
        } else {
          i = new B();
        }
        i.f();
      }