Monthly Archives: January 2009

My mark after the exam …

  1. Exam will be marked from 0 to 10 (being 10 the best mark possible).
  2. Attendance is already marked as a value from 0 to 1 (expressed as a %).
  3. Assignments have been marked too (0 to 2).

First term mark (again from 0 to 10) we’ll be calculated as:

1st-term = attendance + (9-assignments)*exam/10 + assignments

Same process will hold for the second term. Course mark will be obtained by averaging both terms. There is not a minimum average you need to obtain on each term, but course average needs to be equal or higher than 5 points to pass the course.

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To know more about threads synchronization


Assignment#2 presented you an scenario where multiple threads handle one client each. However, given that each thread has to send a text line to each any other thread’s client, a synchronization problem may happen.

If all you use to test server’s code is a few telnet clients running on your computer, it is difficult you see any problem (as only one client is typing at a time). If you’re interested on researching this topic further, you may use IMTester.java code. This program will throw one hundred simultaneous clients to your server, each one sending on hundred text lines and then the mandatory quit command.

You may well see errors like this “Exception in thread “Thread-85” java.util.ConcurrentModificationException” on the server program. It basically means that an object was modified while others were reading from it (not a good thing). While I was not interested on raising this topic of synchronization, some of you pointed it out, so I cannot avoid explaining a bit more.

The main problem happens because the list of clients may change while one (or more) thread is iterating through it. If the client list changes while messages are being sent to other clients it is possible that either a client is missing the message or that a message is attempted to be delivered to client who is gone.

The solution is pretty simple but not obvious, and most of you failed to provide a proper one (not that it was a requirement though). I’m including an slightly modified version of my sample implementation for you that addresses the synchronization problem. The basic idea is that looping through client list and removing a client from the list are performed exclusively.

Update: A Vector object should have been used instead of ArrayList (as Vector is Thread safe and ArrayList is not).

Assignment #2 is marked


You should have got an email answer about your work. At any rate, marks are on the usual place (assignments tab). Those with a 0 mark still can try to fix it by Thursday 15th.

If any of you is missing a mark, please let me know.

You can have a look at my implementation.