Since these presentations were spare time hobby - I've decided to share them :) Hopefully someone will find them useful. This part continues 1. part with more design patterns like Command, State, NullObject.

1. Design patterns...Session N#2 Leonid M.

3. State

4. NullObjectPuzzle questions Mini seminar #1: Content

5. Each pattern describes a problem which occursover and over again in our environment, and then describes the core of the solutionto that problem, in such a way that you can use this solution a million timesover, without ever doing it the same way twice Christopher Alexander, Sara Ishikawa, MurraySilverstein, Max Jacobson,Ingrid Fiksdahl-King, and ShlomoAngel.A Pattern Language. Oxford UniversityPress, NewYork, 1977. Design Pattern Repetitioest mater studiorum

6. Command pattern

7. Synonyms: Action, Transaction Intent: Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. Command

8. There are two extremes that a programmer must avoid when using this pattern: The command is just a link between the receiver and the actions that carry out the request The command implements everything itself, without sending anything to the receiver. We must always keep in mind the fact that the receiver is the one who knows how to perform the operations needed, the purpose of the command being to help the client to delegate its request quickly and to make sure the command ends up where it should. The intelligence of a command

9. Command decouples the object that invokes the operation from the one that knows how to perform it. Commands are first-class objects. They can be manipulated and extended like any other object. You can assemble commands into a composite command. It's easy to add new Commands, because you don't have to change existing classes. Consenquencies

10. Decoupling Menu Application add(Document) Menu Item onClick() Save Command document->save() execute() command->execute() Action

11. Code next door (Swing) import java.awt.Component; import java.awt.event.ActionEvent; import javax.swing.AbstractAction; import javax.swing.JFrame; import javax.swing.JMenu; import javax.swing.JMenuBar; import javax.swing.JOptionPane; class ExitAction extends AbstractAction { public ExitAction() { super("exit"); } public void actionPerformed(ActionEvent e) { System.exit(0); } } class ShowDialogAction extends AbstractAction { public ShowDialogAction() { super("show dialog"); } public void actionPerformed(ActionEvent e) { JOptionPane.showMessageDialog((Component) e.getSource(), "An action generated this dialog"); } } public class Test extends JFrame { public static void main(String args[]) { Test frame = new Test(); frame.setTitle("Swing Actions"); frame.setSize(500, 400); frame.setLocation(400, 200); frame.show(); } public Test() { JMenuBarmb = new JMenuBar(); JMenufileMenu = new JMenu("File"); fileMenu.add(new ShowDialogAction()); fileMenu.add(new ExitAction()); mb.add(fileMenu); setJMenuBar(mb); } }

12. Undo support Macro-commands Queuing requests Logging requests Specific problems /implementation

13. Undo support -addCommand() -undo() -execute() -canUndo() -undo() Please, undo the last one. It caused problems! Can undo? Take back, undo, place in queue client Command manager

14. Code next door (Swing)#Swing’s undo mechanism class diagram

15. Macro-commands

16. /** * Calculates the proper constraint for a child being added. createAddCommand is called afterwards. */ @Override protected Command getAddCommand(Request generic) { // cast request to ChangeBoundsRequest ChangeBoundsRequest request = (ChangeBoundsRequest) generic; // get all Edit Parts of request List editparts = request.getEditParts(); // create a new CompoundCommand CompoundCommand res = new CompoundCommand(); Rectangle r; Object constraint; // for each Edit Part set constraint for (IteratorepIt = editparts.iterator(); epIt.hasNext();) { GraphicalEditPart part = (GraphicalEditPart) epIt.next(); r = part.getFigure().getBounds().getCopy(); // convert r to absolute from childpart figure part.getFigure().translateToAbsolute(r); r = request.getTransformedRectangle(r); // convert this figure to relative getLayoutContainer().translateToRelative(r); getLayoutContainer().translateFromParent(r); r.translate(getLayoutOrigin().getNegated()); constraint = getConstraintFor(r); if ((part instanceofAttributeEditPart) || (part instanceofMethodEditPart) || (part instanceofSlotEditPart)) { return null; } else { res.add(createAddCommand(part, constraint)); } } return res.unwrap(); } Code next door (Gef, Eclipse)

17. Queuing requests User commands are added to the queue This gives an effective way to limit computations to a fixed numbers of threads Threads removes commands from the queue by on one, calls execute(). Once complete, go back for a new command object

19. Logging requests store() store() load() load() store() load() execute() execute() execute() execute() execute() execute()

21. The Null Object Pattern provides intelligent do nothing behavior, hiding the details from its collaborators.NullObject pattern Invoker <<Object interface>> Object request() RealObject NullObject() request() request() { // do nothing; }

22. Null objects can be used in place of real objects when the object is expected to do nothing. Makes client code simple. (reduces branching) Encapsulates the do nothing code into the null object. Makes the do nothing code in the null object easy to reuse. Makes the do nothing behavior difficult to distribute or mix into the real behavior of several collaborating objects. Can be difficult to implement if various clients do not agree on how the null object should do nothing as when your AbstractObject interface is not well defined. Always acts as a do nothing object. The Null Object does not transform into a Real Object. Consenquencies

23. Null Object and Factory The Null Object design pattern is more likely to be used in conjunction with the Factory pattern. The reason for this is obvious: A Concrete Classes need to be instantiated and then to be served to the client. The client uses the concrete class. The concrete class can be a Real Object or a Null Object. Null Object and Template Method The Template method design pattern need to define an abstract class that define the template and each concrete class implements the steps for the template. If there are cases when sometimes template is called and sometimes not then, in order to avoid the checking a Null Object can be use to implement a Concrete Template that does nothing. Removing old functionality The Null Object can be used to remove old functionality by replacing it with null objects. The big advantage is that the existing code doesn't need to be touched. Specific problems /implementation

25. #emptySet()

26. …NullIterator (jdk, internal structures) Code next door

27. public class NullIterator<E> implements Iterator<E> { @Override public booleanhasNext() { return false; // Null implementation returns false indicating iterator is empty } @Override public E next() { throw new NoSuchElementException("Null iterator doesn't contain anything"); } @Override public void remove() { throw new UnsupportedOperationException("Null iterator doesn't support remove operration"); } } Code sample

28. Application: Email spam filtering system. Both HTML and Text formats should be supported. Multiple spam recognition algorithms are present and several could be used simultaneously. Multiple weekly report approaches should be supported: XML, HTML files on filesystem;+ email  with report attachment. Recall Puzzle (prev. sem.)

30. What new considerations come up to mind?

31. How do these changes impact application?

32. How do these impact possible request changes? Pattern puzzle

33. Questions