SWE/CS 332 In Class Exercise # 4

Goals: More converting preconditions into postconditions

Consider the following three excerpts from the java collection classes documentation:

• Set
  

  Note: Great care must be exercised if mutable objects are used as set 
  elements. The behavior of a set is not specified if the value of an object 
  is changed in a manner that affects equals comparisons while the object 
  is an element in the set. A special case of this prohibition is that it 
  is not permissible for a set to contain itself as an element.
  

• Iterator
  

  The behavior of an iterator is unspecified if the underlying collection is modified 
  while the iteration is in progress in any way other than by calling this method.
  

• HashSet
  

  Note that this implementation is not synchronized. If multiple threads access a hash set concurrently, 
  and at least one of the threads modifies the set, it must be synchronized externally.
  

1. Why are all three of these constraints preconditions?
   
   
   
2. HashSet implements both the Set and Iterator interface (the former directly, and the latter through the iterator() method). Have either of the first two preconditions above been converted into postconditions via the exception handling mechanism?
   
   
   
3. How do you know?
   
   
   
4. Can you guess at the likely reason that these two cases are handled differently?