1. Object Oriented Programming Lecture 8: Introduction to laboratorial exercise – part II, Introduction to GUI frames in Netbeans, Introduction to threads www.hh.se/staff/jebe/oop2006/index.html

2. Last lecture – Short recapitulation  A case study – The Algorithm Animator  Demonstrating practical appliance of The Template pattern  A generic animator template for sort algorithms The Strategy pattern  Decoupling the sort algorithms The Factory Pattern  Decoupling creation of specific sort algorithms  The Adapter pattern Narrowing and conversion of class interfaces

3. Laboratorial Exercise – Part II  A Graphical User Interface for the SearchEngine Making queries  Input in terms keywords Ex. A text field  Search button Searching by logical AND using a string of keywords Menu bar  Fetch doc Fetch and show a doc with a specific title  Add doc Add a document to the engine  Quit

4. GUI Forms in Netbeans  Netbeans visual GUI development Both Swing and AWT supported  How to couple Engine with the GUI? Create a new package for your GUI in the project folder Import the Engine package  Demonstration of Netbeans GUI forms

5. Multi-threaded programming  Most programming languages are sequential single-thread programs  Multi-threaded programs Multiple, simultaneous threads can run either  Sequentially on single processor  Concurrently on multiple processors Scheduled by the operating system

6. Multi-threading in Java  Threads are objects Instances of the java.lang.Thread class  Created by extending Thread  Implementing the Runnable interface  All threads executed by the JVM share memory space Threads can communicate through / access shared variables and objects

7. Example – A Concurrent Counter public class Counter extends Thread{ static int id = 0; int value, bound, delay, myid; public Counter(int start, int stop, int d){ value = start; bound = stop; delay = d; myid = id; ++id; } public void run(){ while(value < bound){ System.out.println("Thread nbr " + myid +": " + value); value++; try{ sleep(delay); } catch(InterruptedException ie){ ie.printStackTrace();} }

8. Creating Multiple Counter Threads Public class Concurrent Counters{ public static void main(String args[]){ for(int i = 0; i<10; i++){ new Counter(0,20,50).start(); } Question:How many Thread s are running?

9. Controlling Threads  Threads are controlled by the JVM Scheduled by priority (an integer value) Threads with equal priority will be executed in arbitrary order  No fairness guarantee  Thread scheduling can also be affected by the programmer Setting thread priorities  Can be done dynamically (at runtime) By blocking or yielding  sleep(); yield;... join();

10. Blocked and Runnable states  Blocked mode In sleep or waiting for a target to finish  sleep(milliseconds); Runnable -> Blocked  join(); Runnable -> Blocked  Runnable mode Ready to scheduled and run  yield(); Runnable -> Runnable  interrupt(); Blocked -> Runnable or Runnable with IR-flag

11. Example 2 – Concurrent Counter public class Counter2 extends Thread{ static int id = 0; int value, bound, delay, myid; public Counter2(int start, int stop, int d){ value = start; bound = stop; delay = d; myid = id; ++id; } public void run(){ while(value < bound){ System.out.println("Thread nbr " + myid +": " + value); value++; /** Go to blocked state **/ yield(); }

12. Example 3 – Concurrent Counter public class Counter extends Thread{ static int id = 0; int value, bound, delay, myid; public Counter(int start, int stop, int d){ value = start; bound = stop; delay = d; myid = id; ++id; } public void run(){ while(value < bound){ System.out.println("Thread nbr " + myid +": " + value); value++; }

13. Multiple Counter Threads – Blocking with join() Public class Concurrent Counter3{ for(int i = 0; i<10; i++){ Counter3 cnt = new Counter3(0,20,50); cnt.start(); try{ cnt.join(); } catch(InterruptedException ie){} } Question: In what order will threads be executed now?

14. Critical regions  Objects that are manipulated by multiple threads constitute a critical region Calls to methods or/and access of shared variables race hazard or race condition between threads  Java provide synchronized to warrant atomic access to critical regions

15. Example of critical region: Bank account public class Account{ private int balance; public Account(){ balance = 0;} public boolean insert(int amount){... } public boolean withDraw(int amount){ if(balance >= amount){ newBalance = balance - amount; balance = newBalance; return true; } else { return false; } }

16. A synchronized withDraw() in Bank account public class Account{ private int balance; public Account(){ balance = 0;} public boolean insert(int amount){... } public synchronized boolean withDraw(int amount){ if(balance >= amount){ newBalance = balance - amount; balance = newBalance; return true; } else { return false; } }

17. Bank account: insert() and withDraw() public class Account{ private int balance; public Account(){ balance = 0;} public synchronized boolean insert(int amount){ int newBalance = balance + amount; balance = newBalance; } public synchronized boolean withDraw(int amount){ if(){ balance >= amount; int newBalance = balance - amount; balance = newBalance; return true; } else { return false; } } Question: Consider both synchronized insert and withDraw, are we safe with this?

18. Providing a lock for balance public class Account{ private int balance; private boolean locked = true; public synchronized boolean lock(){ if(locked) { locked = !locked; return locked; } else { return locked; } } public synchronized void insert(int amount){ while(!lock()){ try{ wait(); } catch(InterruptedException e){ } if(!lock()){ break; } // Got the lock? } int newBalance = balance + amount; balance = newBalance; lock(); // Release lock notifyAll(); //Release locked threads }... public synchronized boolean withDraw(int amount){ while(!lock()){ try{ wait(); } catch(InterruptedException e){ } if(!lock()){ break; } // Got the lock? } if(balance >= amount); int newBalance = balance - amount; balance = newBalance; } lock(); // Release lock notifyAll(); //Release blocked threads return true; }