Assignment 7 Solution

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In this assignment, you will practice concurrent programming using Java threads. You will earn total 120 + 10 extra points = 130 points.


Note 1: This homework set is an individual homework, not a team-based e ort. Discussion of the concept is encouraged, but actual write-up of the solutions must be done individually. Note 2: Turn in on eCampus one le that contains all your java source codes (.java les) and a README le that explains how to compile (using the javac command in the terminal), how to run your program (using the java command in the terminal) and what is an expected output of your code when run.


So, once unzipped, it should expand to a subdirectory yourLastName-yourFirstName-hw7 that contains all of your source les and the README le. No .class les please.


Note 3: All Java code that you submit must compile without any error using javac of Java 8 (or higher version). If your code does not compile, you will likely receive zero points for this assignment.


Note 4: Remember to put the head comment in your les, including your name, your UIN, and acknowledgements of any help received in doing this assignment. Again, remember the honor code.



Important: Before solving these problems, carefully read Chapter 14 (especially, the rst seven sections) of the Java textbook.


Problem 0. (10 Points) README. Explain in a README.txt le how to compile (using the javac command in the terminal) and execute your codes (using the java command in the terminal), and what is an expected output of your codes when run (putting all or some part of the output while you are test running your code into your readme le is also a good idea). It should be detailed enough so that whoever grades your codes can understand what you were doing with your code clearly and should be able to reproduce your tests following what you wrote in it.


Problem 1. (80 points) Printing Server (Modi ed Exercise 14.2, page 345 of the textbook.) Consider the following print server that outputs the requested messages to the computer screen as follows: Client threads invoke the printRequest method to submit the messages (strings) to be output. But all the printRequest method actually does is place the message in the print job queue, and a separate (manager) thread then dequeues messages from the job queue and outputs them to the screen. A crude framework of the PrintServer class can be given as below.




public class PrintServer implements Runnable {


private final Queue<String> requests = new LinkedList<String>(); public PrintServer() {


new Thread(this).start();




public void printRequest(String s) {






public void run() {


for(;;) realPrint(requests.remove());




private void realPrint(String s) {


// do the real work of outputting the string to the screen






The code above does not work as it is. First, you need to come up with an idea how to make the requests job queue shared among the threads. There can be di erent ways to implement the shared job queue, one simplest way would be to make it a static object. Since it is shared among multiple threads, you need to take care of the synchronization issue. You will practice two di erent programming constructs to solve the synchronization issue (see details below).


Second, as it is, when you run it, you will get \NoSuchElement” exception because every thread tries to remove elements from the request queue (without adding anything!). In comparison to the BankAccount example, this can be a similar situation to when overdraft occurs. Thus, this problem can also be resolved by carefully synchronizing the accesses to the shared request queue and checking whether the queue is not empty before trying to remove a message. If the queue is empty, the manager thread needs to wait until client threads add some messages, and the client thread, after adding a message, will let the manager thread know by signaling.


Only the thread whose title is \manager” can remove an element from the queue, and a thread whose title is not \manager” can only become a client.


Please write two versions of PrintServer and one test class as below.


Version 1. (40 points) Use lock and condition objects explicitly (as you have seen in the BankAccount example). For this version, the structure of your code will be similar to the BankAccount example code as follows. In the BankAccount example, the account balance is shared among the withdrawing (who takes the amount from the account) and depositing (who adds the amount to the account) threads, whereas in PrintServer, the request queue is shared among the manager (who takes the messages from the queue) and client (who adds the messages to the queue) threads. Name the class for this version class PrintServerV1. The class header should be: public class PrintServerV1 implements Runnable. Place the class de nition in a le named


Version 2. (30 points) Use synchronized methods (or blocks). The basic idea is the same as in Version 1, but practice di erent programming constructs for synchronization. Name the class for this version class PrintServerV2. The class header should be:




public class PrintServerV2 implements Runnable. Place the class de nition in a le named


Meaningful message composition. (10 points) Make the messages added by the clients easy to read/understand by adding the client ID, a serial number of the message by the client, and its class information (PrintServerV1 or PrintServerV2, you can get this information by invoking getClass() method on the object) as part of the message.



The main method can be as simple as follows:


public static void main(String[] args) { // for PrintServerV1


  • The following invocations of the constructor,


  • the first argument is title and the second argument is ID PrintServerV1 m = new PrintServerV1(“manager”, 1); PrintServerV1 c1 = new PrintServerV1(“client1”, 2); PrintServerV1 c2 = new PrintServerV1(“client2”, 3);


new Thread(m).start(); new Thread(c1).start(); new Thread(c2).start();




Or, if you put the statement new Thread(this).start(); at the end of the constructor code, then the main can be even simpler without the last three lines of creating Thread and starting it.


Exactly the same main() with V1s replaced with V2s can be used as the main() method for PrintServerV2. Include the main method that tests your implementation in each of your class PrintServerV1 and class PrintServerV2.



Problem 2. (40 points) Time and messages. (Modi ed Exercise 14.6, page 358 of the textbook.) Consider a shared counter whose values are non-negative integers, initially zero. A time-printing thread increments the counter by one and prints its value each second from the start of execution. A message-printing thread prints a message every ve seconds. Have the message-printing thread be noti ed by the time-printing thread as each second passes by. Add another message-printing thread that prints a di erent message every eleven seconds. Such addition must be done without modifying the time-printing thread implementation.


Have all involved threads share the counter object that is updated by the time-printing thread every second. The time-printing thread will notify other threads to read the counter object each time it updates the counter, then each message-printing thread will read the counter value and see if its assigned time period has elapsed; if so, it will print its message.


The main method can be as simple as:


public static void main(String[] args) {


Time counter = new Time();


  • counter is shared among all three runnables below TimePrinting tp = new TimePrinting(counter); MessagePrinting mp5 = new MessagePrinting(counter, 5); MessagePrinting mp11 = new MessagePrinting(counter, 11);






Again, instead of creating and starting threads in main(), you can include the task in the constructor of each runnable class by placing new Thread(this).start(); as the last step of the constructor.


Include the main() in the Time class, and the class header of Time should be public class Time. In addition, you will have two classes, TimePrinting and MessagePrinting, both of which implement Runnable. Put them in the same le as class Time in the le. The output should look something like this:




  • 5 second message




  • 5 second message




***** 11 second message 12 13 14 15


  • 5 second message


16 17 18 19 20


  • 5 second message


21 22


***** 11 second message 23 24 25


  • 5 second message


. . .



As summary, your .zip directory should contain the following les, nothing else.


  1. txt


  1. java (contains public class PrintServerV1)


  1. java (contains public class PrintServerV2)


  1. java (contains public class Time, class TimePrinting and class MessagePrinting)







Have fun!