Lab 10     ©2011 Felleisen, Proulx, Chadwick, et. al.

Hashcode, Equality, Maps, and Graphs



In the first part of this lab we will learn how to define the equals and hashCode methods, and how to use the HashMap data structure from the Java Collections Framework.

In the second part we will start working on Graph Traversal algorithms, leveraging our knowledge of the Java libraries. You will finish this problem as a part of your programming assignment.

10.1  HashMap and JUnit

Ultimately, the goal of this part of the lab is to learn to use a professional test harness JUnit. It is completely separate from application code and is designed not only to report the cases when a result differs from an expected value, but also to report any exceptions the program would/should throw. The disadvantage of JUnit is that it uses Java equals methods to compare results, which by default only check for instance identity. In order to effectively use JUnit for testing we need to override equals correctly.

As discussed in lecture, each time we override the equals method we must make sure that the hashCode method is also overridden in a compatible way. That means if two instances are equal under our definition of equals then the hashCode method for both instances must produce the same value.

We start by learning to use HashMap class, then see how we can override the necessary hashCode method. Finally, we also override equals to implement an equality comparison that best suits our needs.

Part 1: Using HashMap

Our goal is to design a program that will display the locations of the capitals of all 48 contiguous US states (on a map) and show us how we can travel between the cities.

The problem traveling between cities can be abstracted to the problem of finding a path in a "network" of nodes connected with links — known in combinatorial mathematics as graph traversal problems. You have already seen some parts (Citys) of this problem in earlier assignments.

The Data

To provide real examples of data the provided code includes the (partly complete) definitions of City and State classes.

  1. Download the files in and create a new project.

  2. The Java files contain an implementation of the Traversal interface by InFileCityTraversal that allows you to read a file of City data. The code in the Examples class loads the city data generated by an InFileCityTraversal into an ArrayList<City>.

    Run the code with some of the city data files (caps.txt, minicities.txt, smallcities.txt).

  3. The Examples class contains data for three New England states (ME, VT, MA) and their capitals. Add the data for the remaining three states: CT, NH, RI. Initialize the lists of neighboring states for each of these, but do not include the neighbors outside of the New England region.

  4. Finally, we look at defining a toString method in both the City and State classes. Object defines a default implementation, but it is of little use (it prints the Object’s class name and integer ID).

    Inspect the implementations of toString for City and State. Comment the method out (or just rename it) and rerun the tests... what happens?

    Note: In the future (i.e., for HWs) you should implement a toString method for each class you create. Make the result readable so the fields are clearly destinguished.

We now have all the data we need to proceed with learning about hashCode, equals, and JUnit.

Using a HashMap

The class USMap contains only one field and a constructor. The field is defined as:

   HashMap<City, State> states;

The HashMap is defined to store the values of the type State, each corresponding to a unique key, an instance of a City (its capital).

Note: In reality this would not be a good choice of keys for a HashMap — we do it to illustrate the issues that may come up.

  1. Go to Java documentation and read what is says about HashMap. The two methods you will use the most are put and get.

  2. Define the method initMap in the class Examples that will add the six New England states to a given HashMap (see put).

  3. Test the effect by verifying the size of a HashMap and by checking that it contains at least three of the items you have added. Consult Javadocs to find the methods that allow you to inspect the contents and the size of the HashMap.

Understanding HashMap

We will now experiment with HashMap to understand how changes in the equals method and the hashCode method affect its behavior.

  1. Define a new City instance boston2 initialized with the same values as the original boston. Now put the state MA into the table again using boston2 as the key. The size of the HashMap should now be 7.

  2. Now define an equals method in the class City that compares the City name, state, zip. As we breifly discussed in lecture, start the method with:

       public boolean equals(Object o){
          City c = (City)o;

    If the given object is of the type that cannot be cast to City at runtime, the cast will throw a ClassCastException.

    Now run the same experiment as above (adding MA with boston2). The resulting HashMap still has size 7: even though the two cities are equal, they still produce different hash codes.

  3. Now hide the equals method (comment it out or rename it) and define a hashCode method that produces an integer that is the sum of the hash codes of all the fields in City class (ignoring latitude and longitude).

    Now run the experiment again. The resulting HashMap again has size seven. Even though the two cities produce the same hashCode, the HashMap thinks that they are not the same values.

  4. Un-hide the equals method so that two City objects that we consider to be equal produce the same hash code.

    When you run the experiment again you will see that the size of the HashMap remains the same after we inserted Massachusetts with the boston2 key.

    Note: Read "Effective Java" for a detailed tutorial on overriding equals and hashCode.

Part 2: Introducing JUnit

You will now rewrite all your tests using the JUnit4. In the File menu select New then JUnitTestCase and give your test class a name. The tests for each of the test methods will then become methods similar like:

   /** Testing City toString */
   public void testToString(){
       assertEquals(new City(3301, "Concord", "NH", 71.527734, 43.218525)
                    "new City(03301, Concord, NH, 71.527734, 43.218525)");
       assertEquals(new City(4330, "Augusta", "ME", 69.766548, 44.323228)
                    "new City(04330, Augusta, ME, 69.766548, 44.323228)");


We see that assertEquals calls are basically the same as the test methods for our tester library. Right click on the test class and select Run As, JUnit Test.

Try to see what happens when some of the tests fail, when a test throws an exception, and finally, make sure that in the end all tests succeed.

Ask for help and try things out — make sure you can use JUnit, so you will not run into problems when completing the assignment and your final project.


Try to get as much as possible during the lab. Ask questions when you do not understand something. The first part of the next assignment asks you to hand in a complete solution to the next part of this lab.

10.2  Stacks, Queues, Priority Queues, LinkedLists, and Vectors

Look up the documentation for the following Java classes and interfaces: Stack, Queue, PriorityQueue, List, LinkedList, and Vector. Identify which of them represent interfaces, which represent abstract classes, and which provide concrete implementations you can use in your programs.

Stacks and Queues: Finding a Path

The goal of this exercise is to use the Java libraries to do the work for us. We want to compute a path from one city to another, in a graph that represents the 48 contiguous US states. Start a new project GraphAlgorithms. You will be able to reuse some of what you have done before for the problems that refered to the US cities, but we are starting anew with more effective use of the Java libraries and a better organization of the data.

Download into a new project and run the tests. To help you focus on the interesting parts, we have given you the following classes:

  1. City that represents a capital of a state. It includes the location given as latitude and longitude, as well as methods that compute the location of the city on a Scene of size 400 x 400.

  2. State that represent a state. Its fields are the name of the state (the two letter abbreviation, the capital City and an ArrayList of the names of the neighboring states.

  3. USMap that represents the whole graph - the 48 US capitals and the connections to the neighboring states. This class already has the code that will initialize it with the necessary data.

  1. Start by looking at the representation of the graph of the US. It represents the graph of states as a HashMap<String, State>, that makes it very easy to find a state and its neighbors.

    Note: (this is not important) The method makeStates uses a different technique for initializing an entire ArrayList to the given list of data. You do not need to understand how it is done. At some later time you may want to trace through JavaDocs to understand how this is accomplished,

  2. In looking for a path from one city to another we keep track of the visited States. For each state we visit we also remember the state we came from and the distance we have traveled so far. Design a class FromTo that will represent this information. Because all information about the capitals of all states is already recorded in the class USMap, you only need to record the names of the states. However, include the distance we have traveled from the origin, not the distance between the two states represented.

  3. We now start defining the classes we will need to implement the Graph Traversal Algorithms. We need to keep track of the USMap, the visited states, and a To-Do-List of states to visit. We start with the visited states:

    Define the class Path that keeps track of the visited states using a HashMap. Use the visited state’s name as the Key and the instance of your FromTo class as the Value. So, for example, we may have the following information about states and traveling between them:

       MA - visited first: came from "", distance 0
       NY - we came from MA, distance 130
       NH - we came from MA, distance 60
       VT - we came from NH, distance 60 + 70
       NJ - we came from NY, distance 130 + 100
       PA - we came from NJ, distance 130 + 100 + 90

    Make sure you include the above example in your tests. (The distances you get may be different from the ones we gave you — the given classes implement the computation of distances and your program should use it.

    The class Path should have a constructor that consumes the String that is the name of the origin and adds the first item to its record of visited states. This first FromTo object should have the origin set to the empty String, the distance set to 0, and the destination to be the given origin.

  4. In the class Path design the method fromToDist that consumes two Strings that represent the beginning and ending states for one leg of the journey, and the instance of the USMap and produces the distance between them.

  5. In the class Path design the method add that consumes two Strings that represent the beginning and ending states for one leg of the journey, and the instance of the USMap and adds to the Path the appropriate FromTo object: using the ending state as the key, and adding the current distance to the distance we already traveled to get to the beginning state.

  6. In the class Path design the method pathTo that produces an ArrayList of FromTo-s we need to go through to get to the given City. So, for the above example, we would expect the following results:

       pathTo(MA) --> [MA distance 0]
       pathTo(NY) --> [MA distance 0;
                         NY distance 130]
       pathTo(PA) --> [MA distance 0;
                        NY distance 0 + 130;
                         NJ distance 0 + 130 + 100;
                          PA distance 0 + 130 + 100 + 90]

  7. In the class Path design the method contains that determines whether the state given as String is in this Path.

  8. In the class Path design the method directionsFromTo that consumes the state of origin and our desired destination (as two Strings) and produces the travel directions as a String. For example,

       directions("MA", "MA") produces:
          "from MA go to traveling a total of 0 miles"
       directionso("MA", "PA") produces:
          "from MA go to traveling a total of 0 miles
           from MA go to NY traveling a total of 130 miles
           from NY go to NJ traveling a total of 230 miles
           from NJ go to PA traveling a total of 320 miles"

    We now want to keep track of the neighbors of the states we plan to visit soon (the ToDo checklist). So, for example, if we visit MA, we will add to the ToDo checklist all of its neighboring states. However, there are some restrictions. We do not add a neighbor to the checklist if it is already in the Path.

    The interface ToDo describes the desired behavior:

       interface ToDo{
         /** Add a new neighbor to the ToDo checklist
          * @param state the state whose neighbors we should add
          * @param path the path that has been already traveled
         public void add(String state, Path path);
         /** Remove a state from the ToDo checklist
          *    throw an exception if the checklist is empty
          * @return next state to be visited
         public String remove();
         /** Is this ToDo list empty?
          * @return true if there are no more states to visit
         public boolean isEmpty();

  9. Define the class ToDoStack that keeps track of the neighbors to visit soon that uses the Java Stack class to implement the ToDo interface as a stack.

  10. Define the class ToDoQueue that keeps track of the neighbors to visit soon that uses the Java LinkedList class to implement the ToDo interface as a queue.

The ground work you have done here provides all the parts you need for implementing two different graph traversal algortihms Breadth-First Search and Depth-First Search. You will finish this work in the assignment for this week.

Last modified: Tuesday, March 22nd, 2011 1:34:58am