1. I Like Ike I have always found that plans are useless, but planning is indispensable

2. Computer Science 313 – Advanced Programming Topics

3. Command Pattern Intent  Encapsulate operation as an object  Buffering or logging operations possible  Supports reversal (undo) of operations  Extend, combine ( party-mode ), modify operations  Decouples action invocation from its execution  Classes can be independent of operation executed

4. Command Pattern Example public class Quitter implements ActionListener { public void actionPerformed(ActionEvent e) { System.exit(0); } } public class Bob extends JFrame { JMenuItem quit = new JMenuItem(“Quit”); quit.addActionItem(new Quitter()); } public class Bob Client

5. Command Pattern Example public class Quitter implements ActionListener { public void actionPerformed(ActionEvent e) { System.exit(0); } } public class Bob extends JFrame { JMenuItem quit = new JMenuItem(“Quit”); quit.addActionItem(new Quitter()); } implements ActionListener { Command

6. Command Pattern Example public class Quitter implements ActionListener { public void actionPerformed(ActionEvent e) { System.exit(0); } } public class Bob extends JFrame { JMenuItem quit = new JMenuItem(“Quit”); quit.addActionItem(new Quitter()); } public class Quitter ConcreteCommand

7. Command Pattern Example public class Quitter implements ActionListener { public void actionPerformed(ActionEvent e) { System.exit(0); } } public class Bob extends JFrame { JMenuItem quit = new JMenuItem(“Quit”); quit.addActionItem(new Quitter()); } JMenuItem quit = new JMenuItem(“Quit”); quit.addActionitem(new Quitter()); Invoker

8. Command Pattern Example public class Quitter implements ActionListener { public void actionPerformed(ActionEvent e) { System.exit(0); } } public class Bob extends JFrame { JMenuItem quit = new JMenuItem(“Quit”); quit.addActionItem(new Quitter()); } System.exit(0); Receiver

9. Delaying Gratification  Command pattern encapsulates actions in objects  Enables queuing commands  Program produces Command s placed on queue  Separate part executes & consumes the Command s  Often used to delay actual execution of action  Sometimes see delay from pushing button to action  Bad programs can prevent further action executions  Remember, often does not mean always

10. Information Logs

11. Keeping a Log  Several ways to log each executed Command  If queued, print Command before executing  Rely on Command to print self in execute()‏  Make Invoker call store() method in Command  Which is the best choice depends on  How the system is implemented  What and when you want details logged  Which programmers are most reliable

12. Improving Performance  Modern CPUs contain multiple cores  Each core is complete execution unit  Multiple tasks can be performed at once  Should write programs to take advantage of this  Number of cores varies by processor  Slightly older machines are single core  Dual-core, quad-core, & more also exist  Performs best with one job per core  But how to scale to take advantage of this?

13. Commands to the Rescue

14. How Thread Pools Work  Creates pool of threads to perform work  Each thread executes one command at a time  Completes one, then looks to execute another  Creating thread per core optimizes performance  Need to write ConcreteCommand classes  Can be any class implementing Runnable  Invoker calls run() method in instances  Perfect example of Command Pattern

15. Thread Pool Example public class Lin implements Runnable { public void run() { for (long num = 0; num < 50; num++) System.out.println(“Lin: ” + num); } } public class Exp implements Runnable { public void run() { for (long num = 1; num < 2 50 ; num*=2) { System.out.println(“Exp: ” + num); } }

16. Thread Pool Example public static void main(String[] args){ Runnable command1 = new Lin(); Runnable command2 = new Lin(); Runnable command3 = new Exp(); ExecutorService threadExecutor = Executors.newFixedThreadPool(2); threadExecutor.execute(command1); threadExecutor.execute(command2); threadExecutor.execute(command3); threadExecutor.shutdown(); }

17. Thread Pool Results  Thread pool started with 1, 2, & 3 threads:

18. Transaction-Based System  Databases also big users of command pattern  Normal programs can easily replicate actions  Typically work using transactions  Gather set of changes into single action  Atomically perform all changes in the set  Either ALL changes made or NO changes made

19. Transaction-Based System  Transactions are just a party !  Also very, very hot area of research  Many organizations looking to generalize this  Example of MacroCommand s in wide use  Command places individual changes into group  Apply changes when MacroCommand executed  Add checks ensuring one set changed at a time

20. For Next Class  Lab #6 now available on web/Angel  No class on Friday  Spend the time working on the lab  Friday’s lecture held over until Monday  Will discuss the how code actually compiled  What is SSA? What does it do? How does it work?  Why does this mean long convoluted lines are silly?  How does it help eliminate duplicate code?