# CPSC 250L Lab 9 Networking solution

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## Description

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1 Introduction
This lab introduces basic network I/O. Specifically, in this lab, you will implement a server
that will interact with a client. To do so, you will use sockets. You are able to read from and
write to sockets the same way you do so with files. Specifically, look up the ServerSocket
and Socket classes in the Java API documentation.
2 Exercises
Fork and clone the cpsc250l-lab09 repository in the cpsc250-students group. Your commit quota for this lab is 3. That is, you must commit 3 times in order to receive full credit
for this lab.
2.1 GuessingServer
Exercise 1
Imagine a simple game in which you are asked to guess a number between 0 and 300. You
c Christopher Newport University, 2016
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have 9 tries to guess this number, and you are told either “high” or “low” after each guess
(meaning that your guess is either higher or lower than the number you are trying to guess).
You win the game if you find the number in 9 guesses or less.
Fortunately, we can win the game each time if we use a binary search algorithm: choose
the value in the middle of a sorted list; if it is higher than the number we are guessing then
our number is on the first half of the list; if it is lower, then our number is on the second
half of the list. By subdividing the list in two, we are assured to will find our number in at
most dlog2(n)e tries (where n is the size of the list).
To see how a binary search works, let us imagine that we are guessing a number between 37
and 337. This is a list with 300 values, which means we can guess the number in 9 tries or
less. Following the binary search algorithm, we choose the value in the middle of the list (in
this case 187) as our first guess.
We are told that the number is “low” thus we keep searching on the second half of the
list. Figure 1 shows the different binary search iterations: iteration 1 has values 37 to 337;
we chose the middle value (187) and were told that it was low. This means that for the
next iteration (iteration 2), we will choose the second half of the list (making 187 the lower
boundary) and select its middle value (262). Iterations continue until identifying that the
number to guess is 206.
Iteration Lower Bound Upper Bound Midpoint (Guess) Response
1 37 337 187 “low”
2 187 337 262 “high”
3 187 262 224 “high”
4 187 224 205 “low”
5 205 224 214 “high”
6 205 214 209 “high”
7 205 209 207 “high”
8 205 207 206 “won”
Figure 1: Binary search iterations to find a number between 37 and 337.
In this exercise, you will create a server program that guesses numbers via binary search.
The game is played each time a client contacts the server. The client will begin the game
by sending a message of the form “l h” where l and h represent the initial lower and upper
bounds respectively. Afterwards, you will respond with the midpoint of these numbers
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((l + h)/2) and the client will tell you if guessed correctly (you receive a “won” message), if
your guess is too high (you receive a “high” message), if your guess is too low (you receive
a “low” message), or if you ran out of guesses (you receive a “lost” message). If you won
or lost, you terminate the game immediately. Otherwise, you set the appropriate bound to
the midpoint and guess again. You terminate the server if and only if you receive “SHUT
DOWN” as the initial message in any game. This means, that the server should be able to
play with any number of sequential clients. A diagram of the game protocol is depicted in
Figure 2.
Figure 2: The guessing game message protocol.
Proceed by doing as follows.
1. Create a class called GuessingServer. This class should:
• create a server socket that listens for clients on port 5150;
• follow the algorithm given in the preceding paragraph.
2. Test your code against GuessingServerTest.
Exercise 1 Complete
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Run:
git commit -m “Completed exercise 1”
git push origin master
3 Common Mistakes
The solutions to common mistakes are as follows.
1. Do not shutdown the server until a client sends the “SHUT DOWN” message.
2. Be sure to close the Socket object at the end of each game.
3. Open the PrintWriter as follows.
Code:
PrintWriter toCli = new PrintWriter(client.getOutputStream(), true)
This enables auto flush so that messages can be sent immediately, as opposed to waiting
until there is enough data for a “worthwhile” transmission.
4. Do not stop the server when debugging or after errors. If you stop it then port 5150
will not be available the next time you run the program. If you want to stop it, you
need to send the “SHUT DOWN” message.
5. Use many System.out.println calls when debugging. This will make it easier to see
what error is occurring.
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