Comp201: Principles of Object-Oriented Programming I
Spring 2006 -- Union Design Pattern: Problem Answers   

 

Problem 11:  Not as written, because bop is a variable of type IBinaryOp (i.e. staticly typed as such), which and does not necessarily reference an object of type AddOp. to which aop must reference. That is, a variable of the type of the superclas can always reference an object of the type of any subclass, but a variable of the type of a particular subclass cannot necessarily reference something of the type of its superclass. Another way of saying this is that a superset contains its subsets but not the other way around. The above assignment will cause a compile time error because the compiler cannot know if the assignment is possible. An explicit cast is required to supress the compile time error (aop = (AddOp) bop), but may trigger a run-time error if indeed the instance referred to by bop is not of type AddOp.   Return

 

Problem 10:  Yes! bop is a variable of type IBinaryOp, and aop is defined as referencing an AddOp object, which is an IBinaryOp.  Return  

 

Problem 9: No, because aop is a variable of type AddOp , and MultIOp is not an AddOp, so aop cannot reference an instance of MultOp.   Return  

 

Problem 8:  Yes, because aop is a variable of type AddOp, and thus can reference an instance of the same type.  Return

 

Problem 7: No, because bop is a variable of type IBinaryOp , which is not defined as having a getDescription method. This is true even if bop references an object of type MultOp. This is because the static typing of bop tells the compiler that it references an IBinaryOp, not the particular concrete type of the object it currently references. If we had MultOp mop = new MultOp(), then mop.getDescription() is perfectly legal.  Return

 

Problem 6: It is impossible to tell because it depends on the exact type of the object instance to which myOp refers.   Return

 

Problem 5: The result is 15 because bop now refers to an MultOp instance, whose apply method multiplies its two input values.  Return

 

Problem 4:  The result is 8 because bop refers to an AddOp instance, whose apply method adds its two input values.  Return

 

Problem 3: Yes, for the same reasons as the previous exercise! MultOp is an concrete class and can be instantiated. MultOp is an IBinaryOp, so bop can reference it.  Return

 

Problem 2:  Yes! AddOp is an concrete class and can be instantiated. AddOp is an IBinaryOp (technically, AddOp implements the IBinaryOpinterface), so bop can reference it.   Return

 

Problem 1: No, it won't compile. IBinaryOp is an interface and does not specify any actual executable code, so it cannot be instantiated.   Return

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Last Revised Thursday, 03-Jun-2010 09:50:14 CDT

©2006 Stephen Wong and Dung Nguyen