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CMSC 132:
Object-Oriented Programming II
Project – Terp Idol
Department of Computer Science
University of Maryland, College Park
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Overview
Goal
Implement online voting for a Terp Idol contest
Requires knowledge of
Networking Streams Multithreading
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Terp Idol Contest Rules
The contest consists of a number of rounds
Each round begins with a number of contestants
Voters vote for individual contestants
As many times as they wish
At end of round, contestant w/ fewest votes
eliminated
In case of ties, contestant whose name is
alphabetically first is eliminated
Votes for remaining contestants are reset to 0 at
the beginning of a new round
Last remaining contestant is the winner
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Terp Idol Contest Example
Round 1
Votes – Bart, Lisa, Homer, Lisa, Bart, Lisa Tally – Bart 2, Homer 1, Lisa 3 Homer eliminated
Round 2
Votes – Lisa, Lisa, Bart, Lisa Tally – Bart 1, Lisa 3 Bart eliminated
Round 3
Lisa wins (only contestant)
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Terp Idol Online Voting
Requirements
Must use a client-server model Use threads to handle multiple simultaneous clients Use synchronization to avoid data races
Classes
Server, Client ContestServer, IdolClient, IdolCommandProcessor
General approach
- Set up server
- Set up clients
- Handle client-server requests
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1) Set Up Server
Server class
Supports multiple Clients concurrently Put each Client connection in separate thread Handles ping & disconnect
ContestServer class extends Server
Starts server in separate (daemon) thread
IdolCommandProcessor
Implements processing of commands
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- Set Up Clients
Client class
Creates connection to Server Sets up communication streams Handles ping & disconnect
IdolClient
Support Judges and Voters Different types of requests shown in the table
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Requests & Responses
NEW ROUND NEW ROUND
TALLY TALLY <names & # votes>
INVITE INVITE Judge
IMAGES IMAGES
LOOKUP LOOKUP
VOTE VOTE
CONTESTANTS CONTESTANTS
Voter
INVALID REQUEST
DISCONNECT
PING SERVER RUNNING Client
Sent by Client Sends Server Returns
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- Handle Client-Server Requests
IdolCommandProcessor
Implements processing of commands All commands in the form of Strings
Note
Recommend DataInputStream / DataOutputStream To handle newline characters in data
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Other Relevant Topics
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Client Programming
Basic steps
- Determine server location – IP address & port
- Open network connection to server
- Write data to server (request)
- Read data from server (response)
- Close network connection
- Stop client
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Simple Server Programming
Basic steps
- Determine server location - port (& IP address)
- Create ServerSocket to listen for connections
- Loop while (true) { Accept network connection from client Read data from client (request) Write data to client (response) Close network connection to client }
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Creating Threads in Java
Example
public class MyT implements Runnable {
public void run( ) { … // work for thread }
}
Thread t = new Thread(new MyT( )); // create thread t.start(); // begin running thread … // thread executing in parallel
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Lock
Definition
Entity can be held by only one thread at a time
Properties
A type of synchronization Used to enforce mutual exclusion Thread can acquire / release locks Thread will wait to acquire lock (stop execution) If lock held by another thread
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Lock Example
public class DataRace extends Thread { static int common = 0; static Object o; // all threads use o’s lock public void run() { synchronized(o) { // single thread at once int local = common; // data race eliminated local = local + 1; common = local; } } public static void main(String[] args) { o = new Object(); … } } 22
Stream Input/Output
Stream
A connection carrying a sequence of data (ordered sequence of bytes)
Streams can be associated with
Files, memory, other Strings
Many Java classes for handling streams
Data consisting of characters (e.g., text files) Data consisting of raw bytes (e.g., binary files) Can buffer information
Combining different classes
Can define stream with desired characteristics
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Using Streams
Opening a stream
Connects program to external data Location of stream specified at opening Only need to refer to stream
Usage
- import java.io.* ;
- Open stream connection
- Use stream → read and / or write Catch exceptions if needed
- Close stream
Examples
See fileExamples package 24
Scanner Class
Scanner
Read primitive types & strings from input stream Including System.in (standard input) Provides methods to treat input as String, Integer… Supports String nextLine( ), int nextInt( )… Throws InputMismatchException if wrong format
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Scanner Class Examples
Example 1
// old approach to scanning input BufferedReader br = new BufferedReader( new InputStreamReader(System.in)); String name = br.readLine( ); // new approach using scanner Scanner in = new Scanner(System.in); String name = in.nextLine( ); int x = in.nextInt( );
Example 2
See ScannerExample.java Note use of printf
