CMPS 144L Fall 2023
Lab #8 (Oct. 19)

Activity #1: Point Sorting

Five Java classes are involved, three of which are provided:

• Point: An instance of this class represents a point on the cartesian plane.
• PointSorter: An abstract class that includes a sort() method that is intended to sort an array containing elements of type Point. Its lessThan() method is abstract, the intent being that each of its child classes will provide its own version of that method for the purpose of defining an ordering on the set of points on the plane. The sort() method, applied to an instance of such a child class, would then sort the specified array into ascending order with respect to the ordering defined by that child class's lessThan() method.
• PointSorterTester: An application program that the student can use for testing the Java classes that they are to develop, which are the next two listed.
• PointSorterByX: To be developed by the student, it is a child class of PointSorter having the property that, when its (inherited) sort() method is invoked, the points in the specified array are rearranged to be in ascending order according to their x-coordinates.
• PointSorterByDistFromOrig: To be developed by the student, it is a child class of PointSorter having the property that, when its (inherited) sort() method is invoked, the points in the specified array are rearranged to be in ascending order according to their distances from the origin.

The PointSorterTester program makes use of a "hard-coded" array of Point values. A successful run of that program will produce this output:

Array before sorting: [(3.5,6.2), (-4.8,0.4), (0.0,0.0), (-12.5,1.75), (-2.0,-3.5), (0.5,6.3), (8.25,4.7), (1.25,2.0)] After sorting by x-coordinates [(-12.5,1.75), (-4.8,0.4), (-2.0,-3.5), (0.0,0.0), (0.5,6.3), (1.25,2.0), (3.5,6.2), (8.25,4.7)] That is correct! After sorting by distances from origin [(0.0,0.0), (1.25,2.0), (-2.0,-3.5), (-4.8,0.4), (0.5,6.3), (3.5,6.2), (8.25,4.7), (-12.5,1.75)] That is correct! Goodbye.

If the sort() method does not work as intended, the That is correct! message will be replaced by one or more messages indicating an error.

Activity #2: Counter Who Remembers Max

Three Java classes are involved, two of which is provided in full:

• Counter: An instance of this class represents a Counter.
• CounterWhoRemembersMax: An instance of this class, which is a child of Counter, has (obviously) all the capabilities of an instance of its parent class, but in addition the ability to report the maximum count value it has ever reached.

  The student is to finish the development of this class.

• CWRM_Tester: This application program is for the purpose of testing the CounterWhoRemembersMax class. It expects to receive as a command-line argument (or what jGrasp calls a run argument) a string of I's and D's, standing, respectively, for "increment" and "decrement". The program creates an instance of the CounterWhoRemembersMax class and then makes a series of calls upon its increment() and decrement() methods corresponding to that string of I's and D's. After each such call, the result of calling the toString() method upon the counter is printed. The user should be able to tell, from the program's output, whether the counter is behaving as intended.

To illustrate what the tester program does, here is an example run. Notice that the user supplied the string "DDIIIIDDIDIIIDDDDI" to it as the command line argument.

$ java CWRM_Tester DDIIIIDDIDIIIDDDDI Initially: Count value: 0; Max reached: 0 After decrementing: Count value: -1; Max reached: 0 After decrementing: Count value: -2; Max reached: 0 After incrementing: Count value: -1; Max reached: 0 After incrementing: Count value: 0; Max reached: 0 After incrementing: Count value: 1; Max reached: 1 After incrementing: Count value: 2; Max reached: 2 After decrementing: Count value: 1; Max reached: 2 After decrementing: Count value: 0; Max reached: 2 After incrementing: Count value: 1; Max reached: 2 After decrementing: Count value: 0; Max reached: 2 After incrementing: Count value: 1; Max reached: 2 After incrementing: Count value: 2; Max reached: 2 After incrementing: Count value: 3; Max reached: 3 After decrementing: Count value: 2; Max reached: 3 After decrementing: Count value: 1; Max reached: 3 After decrementing: Count value: 0; Max reached: 3 After decrementing: Count value: -1; Max reached: 3 After incrementing: Count value: 0; Max reached: 3

Activity #3: Red/Blue Partitioning

This activity is to solve Problem 2 on the midterm exam. Two Java classes are involved, the second of which is provided in full:

• RedBluePartitioner: The student is to complete the partition() method, which rearranges the elements of a given array so that all the RED elements come before the BLUE ones. For simplicity, the concepts of RED and BLUE are represented by the char values 'R' and 'B'. All modifications to the array must be made using the swap() method.

  The loop in the partition() method must have as an invariant these conditions:

  • 0 ≤ j ≤ k ≤ N
  • Every element in A[j..k) is RED
  • Every element in A[k..N) is BLUE

  where N is shorthand for A.length. As a diagram, that looks like this:

  0 j k N +------------+-----------+------------+ A | ?? | all RED | all BLUE | && 0<=j<=k<=N +------------+-----------+------------+

• RBPartTester: Java application for testing the partition() method from the RedBluePartitioner class.

  It expects to receive as a command-line argument (or what jGrasp calls a run argument) a string of R's and B's, standing, respectively, for "RED" and "BLUE".

To illustrate what the tester program does, here is an example run. Notice that the user supplied the string "BBRBRRRBRBRR" to it as the command line argument.

$ java RBPartTester BBRBRRRBRBRR Original Array: BBRBRRRBRBRR After partitioning: Red segment: RRRRRRR Blue segment: BBBBB

The partition() method itself verifies, before and after each of its loop iterations, that the intended loop invariant evaluates to true. If not, an error message is produced.