1. Lecture on Design Patterns: Factory, Command

2. Outline What are design patterns? Why should we study them?
List of all design pattern names and categories
Factory pattern
Command pattern

3. Design challenges Designing software for reuse is hard; one must find:
a good problem decomposition, and the right software abstractions
a design with flexibility, modularity and elegance
designs often emerge from an iterative process (trials and many errors)
successful designs do exist
two designs they are almost never identical
they exhibit some recurring characteristics
The engineering perspective: can designs be described, codified or standardized?
this would short circuit the trial and error phase
produce "better" software faster

4. Design patterns history
the concept of a "pattern" was first expressed in Christopher Alexander's work A Pattern Language in 1977 (2543 patterns)
in 1990 a group called the Gang of Four or "GoF" (Gamma, Helm, Johnson, Vlissides) compile a catalog of design patterns
design pattern: a solution to a common software problem in a context
example: Iterator pattern The Iterator pattern defines an interface that declares methods for sequentially accessing the objects in a collection.

5. More about patterns A pattern describes a recurring software structure
is abstract from concrete design elements such as problem domain, programming language
identifies classes that play a role in the solution to a problem, describes their collaborations and responsibilities
lists implementation trade-offs
patterns are not code or designs; must be instantiated/applied
the software engineer is required to:
evaluate trade-offs and impact of using a pattern in the system at hand
make design and implementation decision how best to apply the pattern, perhaps modify it slightly
implement the pattern in code and combine it with other patterns

6. Benefits of using patterns
patterns are a common design vocabulary
allows engineers to abstract a problem and talk about that abstraction in isolation from its implementation
embodies a culture; domain specific patterns increase design speed
patterns capture design expertise and allow that expertise to be communicated
promotes design reuse and avoid mistakes
improve documentation (less is needed) and understandability (patterns are described well once)

7. Gang of Four (GoF) patterns
Creational Patterns (concerned with abstracting the object-instantiation process)
Factory Method Abstract Factory Singleton
Builder Prototype
Structural Patterns (concerned with how objects/classes can be combined to form larger structures)
Adapter Bridge Composite
Decorator Facade Flyweight
Proxy
Behavioral Patterns (concerned with communication between objects)
Command Interpreter Iterator
Mediator Observer State
Strategy Chain of Responsibility Visitor
Template Method

8. Pattern: Factory (a variation of Factory Method, Abstract Factory)
a class used to create other objects

9. Problem: Bulky GUI code
GUI code to construct many components quickly becomes redundant (here, with menus):
homestarItem = new JMenuItem("Homestar Runner");
homestarItem.addActionListener(this);
viewMenu.add(homestarItem);
crapItem = new JMenuItem("Crappy");
crapItem.addActionListener(this);
viewMenu.add(crapItem);
another example (with buttons):
button1 = new JButton();
button1.addActionListener(this);
button1.setBorderPainted(false);
button2 = new JButton();
button2.addActionListener(this);
button2.setBorderPainted(false);

10. Factory pattern
Factory: a class whose sole job is to easily create and return instances of other classes
a creational pattern; makes it easier to construct complex objects
instead of calling a constructor, use a static method in a "factory" class to set up the object
saves lines and complexity to quickly construct / initialize objects
examples in Java: borders (BorderFactory), key strokes (KeyStroke), network connections (SocketFactory)

11. Using existing factories in Java
setting borders on buttons and panels
use built-in BorderFactory class
myButton.setBorder(
BorderFactory.createRaisedBevelBorder());
setting hot-key "accelerators" on menus
use built-in KeyStroke class
menuItem.setAccelerator( KeyStroke.getKeyStroke('T',
KeyEvent.ALT_MASK));

12. Factory implementation details
when implementing a factory of your own:
the factory itself should not be instantiated
make constructor private
factory only uses static methods to construct components
factory should offer as simple an interface to client code as possible
don't demand lots of arguments; possibly overload factory methods to handle special cases that need more arguments
factories are often designed for reuse on a later project or for general use throughout your system

13. Factory sequence diagram

14. Factory example public class ButtonFactory {
private ButtonFactory() {}
public static JButton createButton( String text, ActionListener listener, Container panel) {
JButton button = new JButton(text);
button.setMnemonic(text.charAt(0));
button.addActionListener(listener);
panel.add(button);
return button; }

15. GoF's variations on Factory
Factory Method pattern: a factory that can be constructed and has an overridable method to create its objects
can be subclassed to make new kinds of factories
Abstract Factory pattern: when the topmost factory class and its creational method are abstract

16. objects that represent actions
Pattern: Command
objects that represent actions

17. Open-closed principle
Open-Closed Principle: Software entities like classes, modules and functions should be open for extension but closed for modifications.
The Open-Closed Principle encourages software developers to design and write code in a fashion that adding new functionality would involve minimal changes to existing code.
most changes will be handled as new methods and new classes
designs following this principle would result in resilient code which does not break on addition of new functionality

18. Bad event-handling code
public class TTTGui implements ActionListener {
...
public void actionPerformed(ActionEvent event) {
if (event.getSource() == view1Item){
// switch to view #1 ...
else { // event source must be view2Item
// switch to view #2 ... }
in this code, the "master" TTT GUI object is in charge of all action events in the UI
is this bad?
what could happen if we add another action event source?

19. Common UI commands
it is common in a GUI to have several ways to activate the same behavior
example: toolbar "Cut" button and "Edit / Cut" menu
this is good ; it makes the program flexible for the user
we'd like to make sure the code implementing these common commands is not duplicated

20. Solution: Action,AbstractAction
Java's Action interface represents a UI command
a UI command has a text name, an icon, an action to run
can define multiple UI widgets that share a common underlying command by attaching the same Action to them
These Swing components support Action
JButton(Action a) JToggleButton(Action a)
JCheckBox(Action a) JMenuItem(Action a)
JRadioButton(Action a)
AbstractAction class implements Action and maintains an internal map of keys to values
each key represents a name of a property of the action (e.g. "Name")
each value represents the value for that property (e.g. "Save Game")
can be used to ensure that all UI components that share a common UI action will have the same text, icon, hotkey

21. ActionListener and Action code
reminder: interface ActionListener
public void actionPerformed(ActionEvent e)
interface Action extends ActionListener
adds property enabled
isEnabled() / setEnabled(boolean)
abstract class AbstractAction implements Action
you must still write actionPerformed

22. AbstractAction members
public class AbstractAction implements Action
public AbstractAction(String name)
public AbstractAction(String name, Icon icon)
public Object getValue(String key)
public boolean isEnabled()
public void putValue(String key, Object value)
public void setEnabled(boolean enabled)
...
AbstractAction object maintains an internal map of keys to values
each key represents a name of a property of the action (e.g. "Name")
each value represents the value for that property (e.g. "Save Game")

23. Using Action, example define a class that extends AbstractAction:
public class CutAction extends AbstractAction {
public CutAction() {
super("Cut", new ImageIcon("cut.gif")); }
public void actionPerformed(ActionEvent e) {
// do the action here ...
create an object of this class, attach it to UI objects:
CutAction cut = new CutAction();
JButton cutButton = new JButton(cut);
JMenuItem cutMItem = new JMenuItem(cut);
now the same action will occur in both places; also, changes to the action will be seen on both widgets:
cut.setEnabled(false);

24. Action and Swing components
Other uses: properties of the action can be set
cut.putValue(Action.SHORT_DESCRIPTION, "Cuts the selected text");
will use this label for the tooltip text
cut.putValue(Action.NAME, "Cut");
will use the text label "Cut" for the name of the command
cut.putValue(Action.MNEMONIC_KEY, new Integer('T'));
will underline 't' in "Cut" as a mnemonic hotkey for the command
cut.putValue(Action. ACCELERATOR_KEY, KeyStroke.getKeyStroke('X', KeyEvent.CTRL_MASK));
will use Ctrl-X as an accelerator hotkey for the command

25. Command pattern Command: an object that represents an action
sometimes called a "functor" to represent an object whose sole goal is to encapsulate one function
Java API examples:
ActionListener, Comparator, Runnable / Thread

26. Command pattern applications
command objects serve as a "model" of commands:
separates the user interface from them
each model can support multiple views
each action can support multiple widgets
use the Command pattern when you want to:
implement a callback function capability
have several UI widgets that cause the same action to occur
specify, queue, and execute requests at different times
support undo and change log operations