cs 180 problem solving and object oriented programming fall 2010 notes for week 13: nov 15-19, 2010...

CS 180 Problem Solving and Object Oriented Programming Fall 2010

Notes for Week 13:Nov 15-19, 2010

Aditya MathurDepartment of Computer SciencePurdue UniversityWest Lafayette, IN, USA

http://www.cs.purdue.edu/homes/apm/courses/CS180Fall2010/

This Week:

11/15 1. Review2. Thread interference3. Synchronization

11/17 1. Review2. Synchronization3. Examples

©Aditya Mathur. CS 180. Fall 2010. Week 13

Readings and Exercises for Week 13

Readings:Chapter: 13.3, 13.4, 13.5; 14.3, 14.4

Exercises: 13.4, 13.9, 13.10

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Revised Project Schedule

Project 4:Due: Deadline move to:Thursday November 18, 2010Lab Help Sessions:

Tuesday November 16. 5:30-7:30pmThursday November 18. 5:30-7:30pm

Project 5: Assigned on Wednesday November 17.Deadline: Tuesday December 7.

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Exam 2: Statistics Part A

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Ave: 22.3Median: 22Max: 30Std: 5.37


Exam 2: Statistics Part B

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Ave: 56.5Median: 59Max: 70STD: 11.5


Exam 2: Common mistakes: Use of setText()

Conversion to string before using it in setText()

long n;

setText(n); // Incorrect

setText(n.toString(); // also incorrect

setText(Long.toString(n)); // correct

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Exam 2: Common mistakes: Declarations

Declaring a widget in main() and using it inside the actionPerformed() method.

This works only if you use inner classes, which some of you have used, but not otherwise.

Why? Because the widget is not visible inside actionPerformed() method.

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Exam 2: Common mistakes: == instead of ==

= is the assignment operator.

== is a comparison operator.

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Exam 2: Common mistakes: == instead of equals()

== works well for primitive types.

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Use equals() when comparing objects such as strings.

Given strings s1 and s2, s1 ==s2 might be false even though the strings are identical.


Exam 2: Common mistakes: Use of length

Apply length to find length of an array. length is an attribute (or a field) of an array.

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Use length() to get the length of a string.


Exam 2: Common mistakes: Missing return

A method that has a return type other than void must return something!

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Exam 2: Use of MouseListener

Not a mistake but ActionListener much easier in this problem.

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Concurrency: Review

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Thread: Typical lifecycle

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Define class that extends Thread.

Create thread.

Start thread.

Wait for thread to terminate.

Get results from thread.

Use results from thread.


Thread: The constructor

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A class that extends Thread generally has a constructor that is

used to pass parameters to a thread as follows.

public Search (String [] a, String s, int start, int end, int tID, ){

// Save parameters for use when the thread executes

x=a;

this.s=s; this.start=start; this.end=end;

this.tID=tID

}


Use of “this”

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public class TestThis extends Thread {

int x, y;

static long z;

public TestThis(int x, int y, long z){

this.x=x; // OK

TestThis.y=y; // Not OK; will not compile; y must be

static

TestThis.z=z; // This is OK.

this.z=z; // OK; static can be referenced from object.

}

}


The run() method

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Called when the thread is started using start().

You can certainly call the run() method directly

from its thread object but then the code in the

object will not execute concurrently with other

thread objects.


Restarting a thread?

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No, it is illegal.

Once a thread has terminated, it is illegal to

restart it.

Try restarting a thread to find out what happens if

you do it!


Thread interleaving, shared data, and races

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Thread 1S11S12S13

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Thread 2S21S22S23

S11 S12 S13 S21 S22 S23

A few possible execution interleaving:

S11 S12 S21 S22 S23 S13

S21 S11 S22 S12 S23 S13

How many possible interleaving?

Interleaving


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Shared data

Thread 1

x=x+1;

Thread 2

x=x+1;

x=x+1

Get value of x;

Add 1 to x;

Save x in memory;

int x=0;


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Interleaving and Shared data: race condition

Executing Thread

x Instruction Effect

1 0 Retrieve value of x Thread 1 has 0 2 0 Retrieve value of x Thread 2 has 01 0 Increment value of x Thread 1 has 12 0 Increment value of x Thread 2 has 11 1 Save value of x in memory Value of x changes to 12 1 Save value of x in memory Value of x changes to 1

Final value of x is 1 and not 2!!


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Example: Airline Seat Reservation

Thread A [Mr J]

Get available seat;

if (seat available)Reserve seat;

elseDisplay message;

Thread B [Ms M]

Get available seat;

if (seat available)Reserve seat;

elseDisplay message;

Seat 3A

Assigned to: None


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Two passengers on the same seat!

Executing Thread

Seat Instruction Effect

1 A Get available seat Thread 1 has seat 3A 2 A Get available seat Thread 2 has seat 3A1 A Is seat available? Yes2 A Is seat available? Yes1 NA-J Reserve 3A J gets 3A2 NA-M Reserve 3A But then M gets 3A!!

Seat 3A

Assigned to: M

A: availableNA-X: Not available, assigned to X


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Race condition: Thread

public class Count extends Thread{ Holder h; int c; public Count(Holder h, int incTimes){ this.h=h; c=incTimes; }public void run(){ for(int i=0; i<c; i++){ h.x=h.x+1; } }}


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Race condition: Holder classpublic class Holder{ int x=0; final static int incCount=10000;

public static void main(String [] arg){ Holder h=new Holder(); // Create Holder object Count t1=new Count (h, incCount); Count t2=new Count (h, incCount); t1.start(); t2.start(); try{ t1.join(); t2.join();

}catch(Exception e){} System.out.println(h.x); }}


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Live Example 1: Unsynchronized method

Live Example 2: Synchronized method


Week 13: November 15-19, 2010Hope you enjoyed this week!

Questions?

Contact your recitation instructor. Make full use of our office hours.

11/17/2010

cs 180 problem solving and object oriented programming fall 2010 notes for week 13: nov 15-19, 2010...

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