MCS-287 Homework 7 (Spring 2006)

Due: May 10, 2006

• Exercise 5.x4:

  1. With a copying garbage collector, why is that most objects that are allocated by a typical program are not copied at all?

  2. With a nongenerational copying garbage collector, why is it that most objects that are copied at all are copied repeatedly?

  3. Briefly explain how a generational copying collector is typically able to collect lots of garbage without doing much copying, and in particular without much repeated copying of the same objects.

• Exercise 6.x1: For each of the following two Java statements, indicate whether it is legal, and if so, explain under what circumstances each exception handler would execute. The first version is

          try {
              System.in.read();
          } catch(java.io.CharConversionException e2){
              System.err.println("Caught a CharConversionException.");
          } catch(java.io.IOException e1){
              System.err.println("Caught an IOException.");
          }
  

  and the second version is

          try {
              System.in.read();
          } catch(java.io.IOException e1){
              System.err.println("Caught an IOException.");
          } catch(java.io.CharConversionException e2){
              System.err.println("Caught a CharConversionException.");
          }
  

• Exercise X.1: For each of the following three programs, answer the following questions:

  1. Does this program illustrate a covariant return type or a contravariant return type?

  2. Is this program legal Java?

  3. Does the subclass satisfy Liskov and Wing's definition of a subtype? Briefly explain.

  Program 1:

  abstract class Producer {
      abstract Object produce(); // returns a new Object each time
  }
  
  class IntegerProducer extends Producer {
      Integer produce(){
          return new Integer(42);
      }
  }
  

  Program 2:

  abstract class IntegerProducer {
      abstract Integer produce(); // returns a new Integer each time
  }
  
  class Producer extends IntegerProducer {
      Object produce(){
          return new Object();
      }
  }
  

  Program 3:

  abstract class Producer {
      abstract Object produce(); // returns a new Object each time
  }
  
  class IntegerProducer extends Producer {
      Integer answer = new Integer(42);
  
      Integer produce(){
          return answer;
      }
  }