Homework 7 (Due 11:59pm Thursday, November 5, 2020)

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Preliminaries

This homework should be done in Full Java (using DrJava, IntelliJ, Eclipse, or a text editor and command line compilation and execution).  The Functional Java language in DrJava regrettably no longer works for more complex OO code such as the visitor pattern.  In this assignment, you will re-implement each of the functions on IntLists assigned in Homework 7 using the visitor pattern.

As before, your program must support the object-oriented formulation of lists of integers defined the composite class hierarchy where

  • IntList is an abstract list of int.
  • EmptyIntList is an IntList
  • ConsIntList(first, rest), where first is an int and rest is an IntList, is an IntList

The Homework Support files IntList.java, IntListVisitor.java, LengthVisitor, ScalarProductVisitor, and IntListTest.java provide a starting point for your code.   Feel free to edit these files and omit files that are not needed in this homework assignment.

Problems

Apply the visitor design pattern to define visitor classes implementing the IntListVistor interface IntList and its subclasses given above to formulate all of the following methods as  visitorsJUnit test class, IntListTest to test all of your new methods in the IntList class.  Use the LengthVisitor example as a guide for defining your new visitor classes.  Augment the test clas IntListTest.java to include test methods for each of your visitor classes.  Confine your documentation to writing contracts (purpose statements in HTDP terminology) for each visitor using javadoc notation (a comment preceding the corresponding definition) beginning  with /** and closing  with */ for each visitor class.  Use the documentation of LengthVisitor in the Homework Support files as an example.

  • (10 pts.) boolean contains(int key)  returns true if key is in the list, false otherwise.  Name you visitor class ContainsVisitor.)
  • (10 pts.) int reverse() constructs a list that is the reversal of this. Name your visitor class ReverseVisitor.  Hint: this computation is faster and simpler if you introduce a help "method" that takes an argument (also a visitor).
  • (10 pts.) int sum()  computes the sum of the elements in the list.  Name your visitor class SumVisitor.
  • (10 pts.) double average() computes the average of the elements in the list; returns 0 if the list is empty.  Name your visitor class AverageVisitor.  Hint: you can cast an int to double by using the prefix casting operator (double).  
  • (10 pts.) IntList notGreaterThan(int bound) returns a list of elements in this list that are less than or equal to bound .  Name your visitor class NotGreaterThanVisitor.
  • (10 pts.) IntList remove(int key) returns a list of all elements in this list that are not equal to key.  Name your visitor class RemoveVisitor
  • (10 pts.) IntList subst(int oldN, int newN)  returns a list of all elements in this list with oldN replaced by newN.  Name your visitor class SubstVisitor
  • (15 pts.) IntList merge(IntList other) merges this list with the input list other, assuming that this list and other are sorted in ascending order. Note that the lists need not have the same length. Name your visitor class MergeVisitor.  Hint: add a "method" mergeHelp(ConsIntList other) that does all of the work if one list is non-empty (a ConsIntList). Only mergeHelp is recursive. Use dynamic dispatch on the list that may be empty. Recall that a.merge(b) is equivalent to b.merge(a).  You can formulate help methods as visitors.
  • (15 pts.) IntList mergeSort().  Leveraging the merge "method" you just wrote (as a visitor), write mergeSort() that sorts an IntList.  Recall that you need to write a help function that splits a list approximately in two.

Testing Tricks

In Racket, the equal? function performs structural equality.  Java does not include such a built-in operation.  For the IntList composite type, we overrode the inherited equals method (trivially defined in class Object) by an equals method that implements structural equality but it is slightly more complex than you might expect.  Recall that the argument passed to equal has type Object.  Hence, we have to worry about the class of the argument; the simple (and IMO best) definition of structural equality is to mandate that objects cannot be equal unless they are instances of the same class.  Study the definition of the equals method in class ConsIntList.  Unfortunately, we can write the body of this method the return of a boolean-valued expression, because Java does not support a notion of local or let at the level of expressions.  So the body is an if statement where explicit return statements in the consequent statement and alternative statement.  Notice that we still programmed in a functional style without any mutation.

To test the computations that yield results of composite type, we can either define structural equality over the composite type (as we did for IntList) or write an intelligible toString method for the composite type (which I strongly recommend for debugging purposes) and compare the toString() representations of the composite type.  But beware that toString() equality may not imply structural equality and vice versa.  You should always endeavor to make them agree.

 
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