Notes: Thursday 17 October 2002

Schedule

• Thursday, October 24: Problem Set 5: Part 1
• There will be no class on Thursday, October 24. You can drop off your PS5: Part 1 outside my office anytime before 5pm.
• Thursday, October 31: Problem Set 5: Part 2

Substitution

   MysteryType1 mt1;
   MysteryType2 mt2;
   MysteryType3 mt3;
   ... (anything could be here)
   mt1 = mt2.m (mt3);

If the Java compiler is happy with this code, which of these are guaranteed to be true:

1. The apparent type of mt2 is MysteryType2
2. At the last statement, the actual type of mt2 is MysteryType2
3. MysteryType2 has a method named m
4. The MysteryType2.m method takes a parameter of type MysteryType3
5. The MysteryType2.m method returns a subtype of MysteryType1
6. After the last statement, the actual type of mt1 is MysteryType1

class A {
   public RA m (PA p) ;
}
class B extends A {
   public RB m (PB a);
}

               Substitution Principle        Eiffel
Parameters     PB >= PA	              PB <= PA
Preconditions  pre_A implies pre_B    pre_B implies pre_A

Result         RB <= RA               RB <= RA
Postconditions post_B implies post_A  post_B implies post_A

#include <stdio.h>

class A {
  public: void other () { printf("is an empty func in A\n"); };
              virtual void other (class A *a) { printf("In A\n"); }
};
class B: public A {
   public: void other (class B *b) { printf("In B\n"); }
 };
class C: public A {
     public: void other (class C *c) { printf("In C\n"); }
 };
void main(void) {
    A a; B b; C c;
    A *aPtr = &a; B *bPtr = &b; C *cPtr = &c;
    aPtr = bPtr;
    aPtr->other(bPtr); 
    bPtr->other();      
}

University of Virginia Department of Computer Science CS 201J: Engineering Software

Sponsored by the National Science Foundation

cs201j-staff@cs.virginia.edu