CSCI 2600 Homework 5: The Marvel Comics Universe solution


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Marvel Comics #1Introduction

Like Homework 4, this homework has two parts. In the first part (Problem 1), you will practice concepts we discussed in class. In the second part, you will put the graph you designed in Homework 4 to use by modeling the Marvel Comics universe. By trying out your ADT in a client application, you will be able to test the usability of your design as well as the correctness, efficiency, and scalability of your implementation.

The application builds a graph containing thousands of nodes and edges. At this size, you may discover performance issues that weren’t revealed by your unit tests on smaller graphs. With a well-designed implementation, your program will run in a matter of seconds. Bugs or less ideal choices of data structures can increase the runtime to anywhere from several minutes to 30 minutes or more. If this is the case you may want to go back and revisit your graph implementation from Homework 4. Remember that different graph representations have widely varying time complexities for various operations and this, not a coding bug, may explain the slowness.

This Marvel dataset was also used by researchers, who published a research paper showing that the graph is strikingly similar to “real-life” social networks.

Problem 1: Written Exercises [10 points]

This part is designed to improve your understanding of function subtyping and true subtyping from lecture. Place your answers to the questions below in file hw5/answers/problem1.txt.

  1. Which of the B.m methods below are function substypes of A.m. For each of the B.m methods answer whether the method would overload or override A.m in Java.
          class A {
            Object m(X y, String s);
          class B extends A {
            X m(X y, String s);
            Y m(Object y, Object s);
            Object m(X y, String s) throws RuntimeException;
            Z m(Y y, String s);        
  2. Given the specifications below, answer whether the extending class is a true subtype of its superclass. Explain your answer.
    1.     class Triangle {
             // modifies: this
             // effects: this_post.a=a, this_post.b=b, this_post.c=c
             void setSides(int a, int b, int c);
          class IsoscelesTriangle extends Triangle {
             // modifies: this
             // effects: this_post.a=a, this_post.b=b, this_post.c=b
             void setSides(int a, int b, int c);
    2.     abstract class Vertebrate extends Animal {   
             // returns: an integer > 0 
             int neckBones();
          class Squid extends Vertebrate {
             // returns: 0 neck bones
             int neckBones();
          class Human extends Vertebrate {
             // returns: 7 neck bones
             int neckBones();
    3.     class Bicycle {
             int cadence;
             int speed;
             int gear;
             // effects: creates a new Bicycle
             Bicycle(int startCadence, int startSpeed, int startGear);
             // modifies: this
             // effects: this_post.cadence=newCadence, this_post.speed=newSpeed, this_post.gear=newGear
             void setParameters(int newCadence, int newSpeed, int newGear);
          class MountainBike extends Bicycle {
             int height;
             // effects: creates a new MountainBike
             MountainBike(int startCadence, int startSpeed, int startGear, int startHeight);
             // modifies: this
             // effects: this_post.height=newHeight
             void setHeight(int newHeight);
    4.     class Account {
             // modifies: this
             // effects: this_post.amount = this_pre.amount + d;
             void deposit(int d);
          /* An account that works safely when multiple transactions attempt deposits simultaneously */
          class ConcurrentAccount extends Account {
             // modifies: this
             // effects: this_post.amount = this_pre.amount + d;
             // throws: AbortException if another transaction is in the process of depositing money
             void deposit(int d);

The MarvelPaths Application

In this application, your graph models a social network among characters in Marvel comic books. Each node in the graph represents a character, and an edge ⟨Char1,Char2⟩ indicates that Char1 appeared in a comic book that Char2 also appeared in. There should be a separate edge for every comic book, labelled with the name of the book. For example, if Zeus and Hercules appeared in (say) five issues of a given series, then Zeus would have five edges to Hercules, and Hercules would have five edges to Zeus.

Your graph should not store reflexive edges from characters to themselves.

You will write a class hw5.MarvelPaths (in file in package hw5) that reads the Marvel data from a file (marvel.csv), builds a graph, and finds paths between characters in the graph. You are not required to write a main method as a driver for your application; nevertheless, we encourage you to do so, both for your own convenience and for the satisfaction that comes with creating a complete, standalone application.

As you complete this assignment, you may need to modify the implementation and perhaps the public interface of your Graph ADT from Homework 4. Briefly document any changes you made and why in hw5/answers/changes.txt (no more than 1-2 sentences per change). If you made no changes, state that explicitly. You don’t need to track and document cosmetic and other minor changes, such as renaming a variable; we are interested in substantial changes to your API or implementation, such as adding a public method or using a different data structure. Describe logical changes rather than precisely how each line of your code was altered. For example, “I switched the data structure for storing all the nodes from a ___ to a ___ because ___” is more helpful than “I changed line 27 from nodes = new ___(); to nodes = new ____();.”

Leave your graph in the hw4 package where it was originally written, even if you modify it for this assignment. There is no need to copy files nor duplicate code! You can just import hw4 and use it in Homework 5. If you do modify your hw4 code, be sure to commit your changes to your repository. (Subclipse easily takes care of this.)

Problem 2: Getting the Marvel Universe Data

Before you get started, obtain the Marvel Universe dataset. Download the file from Store the file in hw5/data/marvel.csv.

IMPORTANT: Do not commit the marvel.csv into your repository! There is a limit on each repository and committing such a large file may break this limit. This is easily taken care of in Subclipse, you can exclude marvel.csv simply by removing the check mark during the commit.

Take a moment to inspect the file. A CSV (“comma-separated value”) file consists of human-readable data delineated by commas, and can be opened in your favorite text editor or Eclipse. (If you don’t have a favorite text editor, Notepad++ is a simple, free, easy-to-use option for Windows.) Each line in marvel.csv is of the form


where character is the name of a character, book is the title of a comic book that the character appeared in, and the two fields are separated by a comma.

Problem 3: Building the Graph [27 points with Problem 4]

The first step in your program is to construct your graph of the Marvel universe from a data file. We have written a class MarvelParser to help you. MarvelParser has one static method, readData(), which reads data from marvel.csv, or any file structured in the same format. readData() creates in-memory data structures: a Set of all characters and a Map from each book to a Set of the characters in that book. These are not the data structures you want, however; you want a Graph.

I have also included a main method which takes the file name as a command-line argument and then calls readData(). To add marvel.csv as a command-line argument in Eclipse, go to Run->Run Configurations. In the Run Configurations window, choose the “Arguments” tab. In “Program Arguments”, type hw5/data/marvel.csv.

Later on when measuring coverage, you can comment out the main method. (As it is never called from the test cases.) If you choose not to use my parser code, you can get rid of the file altogether. You do need to substitute with your own parser code.

You may modify MarvelParser however you wish to fit your implementation. You may change the method signature (parameters and return value) of parseData(), or you might leave parseData() as is and write code that processes its output. The only constraint is that your code needs to take a filename as a parameter so the parser can be reused with any file.

At this point, it’s a good idea to test the parsing and graph-building operation in isolation. Verify that your program builds the graph correctly before you go on. The assignment formally requires this in Problem 5.

Problem 4: Finding Paths [27 points with Problem 3]

The real meat (or tofu) of MarvelPaths is the ability to find paths between two characters in the graph. Given the name of two characters, MarvelPaths searches for and returns a path through the graph connecting them. How the path is subsequently used, or the format in which it is printed out, depends on the requirements of the particular application using MarvelPaths.

Your program should return the shortest path found via breadth-first search (BFS). A BFS from node u to node v visits all of u’s neighbors first, then all of u’s neighbors’ neighbors’, then all of u’s neighbors’ neighbors’, and so on until v is found or all nodes with a path from u have been visited. Below is a general BFS pseudocode algorithm to find the shortest path between two nodes in a graph G. For readability, you should use more descriptive variable names in your actual code than are needed in the pseudocode:

    start = starting node
    dest = destination node
    Q = queue, or "worklist", of nodes to visit: initially empty
    M = map from nodes to paths: initially empty.
        // Each key in M is a visited node.
        // Each value is a path from start to that node.
        // A path is a list; you decide whether it is a list of nodes, or edges,
        // or node data, or edge data, or nodes and edges, or something else.
    Add start to Q
    Add start->[] to M (start mapped to an empty list)
    while Q is not empty:
        dequeue next node n
        if n is dest
            return the path associated with n in M
        for each edge e=⟨n,m⟩:
            if m is not in M, i.e. m has not been visited:
                let p be the path n maps to in M
                let p' be the path formed by appending e to p
                add m->p' to M
                add m to Q
    If the loop terminates, then no path exists from start to dest.
    The implementation should indicate this to the client.

Here are some facts about the algorithm.

  • It is a loop invariant that every element of Q is a key in M
  • If the graph were not a multigraph, the for loop could have been equivalently expressed as for each neighbor m of n:
  • If a path exists from start to dest, then the algorithm returns a shortest path.

Many character pairs will have multiple paths. For grading purposes, your program should return the lexicographically (alphabetically) least path. More precisely, it should pick the lexicographically first character at each next step in the path, and if those characters appear in several comic books together, it should print the lexicographically lowest title of a comic book that they both appear in. The BFS algorithm above can be easily modified to support this ordering: in the for-each loop, visit edges in increasing order of m‘s character name, with edges to the same character visited in increasing order of comic book title. This is not meant to imply that your graph should store data in this order; it is merely a convenience for grading.

Because of this application-specific behavior, you should implement your BFS algorithm in MarvelPaths rather than directly in your graph, as other hypothetical applications that might need BFS probably would not need this special ordering. Further, other applications using the graph ADT might need to use a different search algorithm, so we don’t want to hard-code a particular search algorithm in the graph ADT.

Using the full Marvel dataset, your program must be able to construct the graph and find a path in just a couple of seconds on your PC/laptop and on the Homework Server. When running tests we will set a 60 second timeout for each test suite. Note that if your solution exceeds the limit on the Homework Server, your process is cut then the output file is cut as well resulting in a grade of 0 and an Error message complaining about incorrect formatting of the output file.

Similarly to Homework 4, add an instance field that stores a Graph in MarvelPaths. For testing purposes, we require that you implement the following public methods in MarvelPaths. Otherwise, design class MarvelPaths as you wish and add operations as you wish.

public void createNewGraph(String filename)

The method creates a brand new graph in the instance field in MarvelPaths and populates the graph from filename, where filename is a data file of the format defined for marvel.csv and is located in the hw5/data directory of your project.

public String findPath(String node1, String node2)

Find the shortest path from node1 to node2 in the graph using your breadth-first search algorithm.

Paths should be chosen using the lexicographic ordering described above. If a path is found, the returned String should contain the path in the format below. That is, System.out.println(mp.findPath("CHAR1","CHARN") where mp refers to an instance of MarpvelPaths, prints the following:

path from CHAR1 to CHARN:
CHAR1 to CHAR2 via BOOK1
CHAR2 to CHAR3 via BOOK2

where CHAR1 is the first node listed in the arguments to findPath, CHARN is the second node listed in the arguments, and BOOKK is the title of a book that CHARK and CHARK+1 appeared in.

For example, mp.findPath("Zena","Zeus") will construct and return the String "path from Zena to Zeus:\nZena to Hercules via Book1\nHercules to Zeus via Book2\n". (I’m just making this up to illustrate the placement of newline characters, there is no such a path in the Marvel universe.)

Not all characters may have a path between them. If the user gives two valid node arguments that have no path in the specified graph, output the following as String:

path from CHAR 1 to CHAR N:
no path found

If a character name CHAR was not in the original dataset, simply output:

unknown character CHAR

If neither character is in the original dataset, output the line twice: first for the first node, then for the second one. These should be the only lines your program produces in this case — i.e., do not output the regular “path from …” or “path not found” output too.

What if the user asks for the path from a character in the dataset to itself? A trivially empty path is different from no path at all, so the “no path found” output isn’t appropriate here. But there are no edges to print, either. So you should output the header line

path from C to C:

but nothing else. (Hint: a well-written solution requires no special handling of this case.)

This only applies to characters in the dataset: a request for a path from a character that is not in the dataset to itself should have the the usual “unknown character C” output.

In all cases the string should end with \n (newline), just like in the first case.

Problem 5: Testing Your Solution [12 points]

Because the Marvel graph contains literally thousands of nodes and hundreds of thousands of edges, using it for correctness testing is probably a bad idea. By contrast, using it for scalability testing is a great idea, but should come after correctness testing has been completed using much smaller graphs. In addition, it is important to be able to test your parsing/graph-building and BFS operations in isolation, separately from each other.

You should first write *.csv files in the format defined for marvel.csv to test your MarvelPaths. All these files will go in the hw5/data directory.

Write tests in the regular JUnit test class. Make sure that you handle the edge cases. You will have to specify data files to load in your implementation tests, so make sure you read the File Paths section for information about specifying filenames very carefully.

As in Homework 4, run EclEmma and measure coverage of your tests. We will be measuring coverage in the Homework Server.

Reflection [0.5 point]

Please answer the following questions in a file named reflection.txt in your answers/ directory. Answer briefly, but in enough detail to help you improve your own practice via introspection and to enable us to improve PoS in the future.

  1. In retrospect, what could you have done better to reduce the time you spent solving this assignment?
  2. What could have we done better to improve your learning experience in this assignment?
  3. What do you know now that you wish you had known before beginning the assignment?

Collaboration [0.5 point]

Please answer the following questions in a file named collaboration.txt in your answers/ directory.

The standard integrity policy applies to this assignment.

State whether or not you collaborated with other students. If you did collaborate with other students, state their names and a brief description of how you collaborated.

Grade Breakdown

  • Quality of test suite, percent of your tests passed: 6pts (auto-graded)
  • Quality of test suite, percent coverage: 6pts (auto-graded)
  • Instructor MarvelPaths small tests: 7pts (auto-graded)
  • Instructor MarvelPaths large tests: 8pts (auto-graded)
  • Answers to Problem 1 questions (hw5/answers/problem1.txt): 10pts
  • Changes (hw5/answers/changes.txt): 5pts
  • Code quality (hw5/*.java, specs, rep invariants, AFs, etc.): 7pts
  • Collaboration and reflection: 1pt

Paths to Files

When you use test files in hw5/data, hardcode the relative path in your tests. For example, if you use file testfile1.csv in directory hw5/data, you can load the file using BufferedReader reader = new BufferedReader(new FileReader("hw5/data/testfile1.csv")).

Do not use src/hw5/data/testfile1.csv because the Homework Server assumes relative path "hw5/data/testfile1.csv" and prepending src/ will result in FileNotFoundException and a grade of 0. Because Eclipse searches for files relative from your project directory (which is the default Working Directory), the default compilation configuration does require prepending src/ to the path. To run without having to prepend src, go to Run Configuration -> Arguments, then in Working Directory, click Other, then append src to whatever shows in the Default box. E.g., in my case, I typed ${workspace_loc:csci2600}/src, where ${workspace_loc:csci2600} was the default Working Directory. Relative paths "hw5/data/testfile1.csv" should work with this configuration.

Behavior may vary from one version of Eclipse to another. As long as you hardcode your relative paths starting at hw5/data you will be fine on the Homework Server.



If your program takes an excessively long time to construct the graph for the full Marvel dataset, first make sure that it parses the data and builds the graph correctly for a very small dataset. If it does, here are a few questions to consider:

  • What data structures are you using in your graph? What is their “big-O” runtime? Are there others that are better suited to the purpose?
  • Did you remember to correctly override hashCode() if you overrode equals()?
  • What is the “big-O” runtime of your checkRep() function? Does performance improve if you comment it out?


As always, remember to:

  • Use descriptive variables names (especially in the BFS algorithm) and inline comments as appropriate.
  • Include an abstraction function, representation invariant, and checkRep in all classes that represent an ADT. If a class does not represent an ADT, place a comment that explicitly says so where the AF and RI would normally go. (For example, classes that contain only static methods and are never constructed usually do not represent an ADT.) Please come to office hours if you feel unsure about what counts as an ADT and what doesn’t.

What to Submit

You should add and commit the following files to SVN:

  • hw5/
  • hw5/*.java [Other classes you create, if any (there may be none!)]
  • hw5/answers/problem1.txt
  • hw5/answers/changes.txt
  • hw5/answers/reflection.txt
  • hw5/answers/collaboration.txt
  • hw5/data/*.csv [Your .csv test files. Don’t commit marvel.csv.]
  • hw5/test/* [JUnit tests classes you create]

Additionally, be sure to commit any updates you may have made to the following files, so the staff has the correct version for this assignment:

  • hw4/*.java [Your graph ADT]
  • hw5/


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