CSE373 Fall 2013: Homework 5  Graphs and
Shortest Paths
For this assignment, you will develop a graph representation and use
it to implement Dijkstra's algorithm for finding shortest paths.
Unlike previous assignments, you will use some classes in the Java
standard libraries, gaining valuable experience reading documentation
and understanding provided APIs. Also, part of the grade for your
code will depend on you "protecting the graph abstraction"
leveraging copyincopyout and immutability as we discussed (or will
discuss) in class.
Outline
Due Dates, TurnIn, and Rules
Different for this assignment: You may use anything in the
Java standard collections (or anything else in the standard library)
for any part of this assignment. Take a look at
the Java API
as you are thinking about your solutions. At the very least, look at
the Collection
and List
interfaces to see what operations are allowable on them and what
classes implement those interfaces.
Partner Selection Due 11:00PM Tuesday November 19, 2013
Assignment Due 11:00PM Tuesday November 26, 2013
For this assignment, you may work with one partner. If you
do so, the two of you will turn in only one assignment and, except in
extraordinary situations, receive the same grade (and, if applicable,
have the same number of late days applied). Working with a partner is
optional; it is fine to complete the assignment by yourself.
If you choose to work with a partner, one of you must
complete this web form before the partnerselection due
date. If you choose to work alone, do not complete the form. If you
make a mistake filling out the form, email the course staff.
If you choose to work with a partner, you may divide the work
however you wish, but both partners must understand and be
responsible for everything that is submitted. Beyond working with a
partner, all the
usual collaboration
policies apply.
Read What to Turn In before you submit
— poor submissions may lose points.
Turn in your assignment here
Provided Files
Download these files into a new directory:
 Graph.java  Graph
interface. Do not modify.
 Vertex.java  Vertex class
 Edge.java  Edge class
 MyGraph.java  Implementation of
the Graph interface: you will need to fill in code
here
 Path.java  Class with two
fields for returning the result of a shortestpath computation. Do not modify.
 FindPaths.java  A client of the
graph interface: Needs small additions

vertext.txt and
edge.txt 
an example graph in the correct input format
Programming, Part 1
In this part of the assignment, you will implement a graph
representation that you will use in Part 2. Add code to the
providedbutincomplete MyGraph class to implement
the Graph interface. Do not change the arguments to the
constructor of MyGraph and do not add other constructors.
Otherwise, you are free to add things to
the Vertex, Edge, and MyGraph classes, but
please do not remove code already there and do not
modify Graph.java. You may also create other classes if you
find it helpful.
As always, your code should be correct (implement a graph) and
efficient (in particular, good asymptotic complexity for the requested
operations), so choose a good graph representation for computing
shortest paths in Part 2.
We will also grade your graph representation on how well it protects
its abstraction from bad clients. In particular this means:
 The constructor should check that the arguments make sense and
throw an appropriate exception otherwise. You can define your own
exceptions by making classes like the exception classes we have provided
you in previous homeworks. Here is what we man by make sense:
 The edges should involve only vertices with labels that are
in the vertices of the graph. That is, there should be no edge
from or to a vertex labeled A if there is no vertex
with label A.
 Edge weights should not be negative.
 Do not throw an exception if the collection of
vertices has repeats in it: If two vertices in the collection have
the same label, just ignore the second one encountered as
redundant information.
 Do throw an exception if the collection of edges has the
same directed edge more than once with a different weight.
Remember in a directed graph an edge from A to B is not the
same as an edge from B to A. Do not throw an exception if an edge
appears redundantly with the same weight; just ignore the
redundant edge information.
 As discussed in class, it should not be possible for clients of a
graph to break the abstraction by adding edges, making illegal
weights, etc. So the code most have enough copyincopyout and immutability to
prevent clients from doing such things.
Other useful information:
 The Vertex and Edge classes have already
defined an appropriate equals method (and, therefore, as we
discussed in class, they also define hashCode appropriately).
If you need to decide if two Vertex objects are
"the same", you probably want to use the equals
method and not ==.
 You will likely want some sort
of Map
in your program so you can easily and efficiently look up information
stored about some Vertex. (This would be much more efficient
than, for example, having a Vertex[] and iterating through it
every time you needed to look for a particular Vertex.)
 As you are debugging your program, you may find it useful to print out your data structures. There are toString methods for Edge and Vertex. Remember that things like ArrayLists and Sets can also be printed.
Programming, Part 2
In this part of the assignment, you will use your graph from Part 1 to
compute shortest paths. The MyGraph class has a
method shortestPath you should implement to return the
lowestcost path from its first argument to its second argument.
Return a Path object as follows:
 If there is no path, return null.
 If the start and end vertex are equal, return a path containing
one vertex and a cost of 0.
 Otherwise, the path will contain at least two vertices  the
start and end vertices and any other vertices along the lowestcost
path. The vertices should be in the order they appear on the path.
Because you know the graph contains no negativeweight edges,
Dijkstra's algorithm is what you should implement. Additional implementation notes:
 One convenient way to represent infinity is with Integer.MAX_VALUE.
 Using a priority queue is aboveandbeyond. You are not required
to use a priority queue for this assignment. Feel free to use any
structure you would like to keep track of distances and then search
it to find the one with the smallest distance that is also
unknown.
 You definitely need to be careful to
use equals instead of == to compare Vertex
objects. The way the FindPaths class works (see below) is to
create multiple Vertex objects for the same graph vertex as
it reads input files. You may want to refer to your old notes on the
equals method from CSE143. Remember that equals lets
us compare values (e.g. do two Vertex objects have the same label) as
opposed to just checking if two things refer to the exact same object.
The program in FindPaths.java is mostly provided to
you. When the program begins execution, it reads two data files
and creates a representation of the graph. It then prints out the
graph's vertices and edges, which can be helpful for debugging to help
ensure that the graph has been read and stored properly. Once the
graph has been built, the program loops repeatedly and allows the user
to ask shortestpath questions by entering two vertex names.
The part you need to add is to take these vertex names,
call shortestPath, and print out the result. Your output
should be as follows:
 If the start and end vertices are X and Y, first print a line
Shortest path from X to Y:
 If there is no path from the start to end vertex, print exactly one more line
does not exist
 Else print exactly two more lines. On the first additional line,
print the path with vertices separated by spaces. For example, you
might print
X Foo Bar Baz Y. (Do not print a period, that is just
ending the sentence.) On the second additional line, print the cost
of the path (i.e., just a single number).
The FindPaths code expects two input files in a particular
format. The names of the files are passed as commandline arguments.
The provided files vertex.txt and edge.txt have the
right formate to serve as one (small) example data set where the
vertices are 3letter airport codes. Here is the file format:
 The file of vertices (the first argument to the program) has one
line per vertex and each line contains a string with the name of a
vertex.
 The file of edges (the second argument to the program) has three
lines per directed edge (so lines 13 describe the first edge, lines
46 describe the second edge, etc.) The first line gives the source
vertex. The second line gives the destination vertex. The third line
is a string of digits that give the weight of the edge (this line
should be converted to a number to be stored in the graph).
Note data files represent directed graphs, so if there is an edge
from A to B there may or may not be an edge from B to A. Moreover, if
there is an edge from A to B and an edge from B to A, the edges may or
may not have the same weight.
WriteUp Questions
 Describe the worstcase asymptotic running times of your
methods adjacentVertices, edgeCost,
and shortestPath. In your answers, use E for the
number of edges and V for the number of
vertices. Explain and justify your answers.

Describe how your code ensures the graph abstraction is protected. If
your code makes extra copies of objects for (only) this purpose,
explain why. If not, explain why it is not necessary.
 Describe how you tested your code.
 If you worked with a partner, describe how you worked
together. If you divided up the tasks, explain how you did so. If
you worked on parts together, describe the actual process. Discuss
how much time you worked together and how you spent that time
(planning, coding, testing, ...). Be sure to describe at least one
good thing and one bad thing about the process of working with a
partner.
 If you did any aboveandbeyond, describe what you did.
Above and Beyond
 Find an interesting realworld data set and convert it into the
right format for your program. Describe in your writeup questions
what the data is and what a shortest path means. Turn in your data
set in the right format as two additional files.
 Improve your implementation of Dijkstra's algorithm by using a
priority queue. Note that for Dijkstra's algorithm, we need
to find items in the priority queue and update their priorities
(the decreaseKey operation). We would like to find items in
constant time (and then logarithmic time for changing the priority).
There are various ways to do this, including keeping a backpointer
from each vertex to its entry in the priority queue. Note: If you
implement this above and beyond, you are not required to also
implement Dijkstra's without a priority queue. You may submit the
priority queue version as your only submission, but be sure to
indicate in your writeup that you did this.
 Extend MyGraph with a method for computing minimum
spanning trees using one of the efficient algorithms discussed in
class. Also write a driver program that reads in a graph and prints a
minimum spanning tree. This driver will be much like FindPaths, but
make a separate file and do not prompt the user for vertices or have a
loop  just print one minimum spanning tree. Explain in your
writeup the format of what you print.
What to Turn In
If you work with a partner, only one partner should submit the
files. Be sure to list both partners' names in your files.
You will turn in everything electronically. This should
include all your code files, including the files provided to
you (but you do not need to turn in vertex.txt
and edge.txt). This should also include a file with your
writeup; do not forget to turn in your writeup.