1. Design Patterns, conclusion

2. So Far: Creational Patterns
Factories, Prototype, Singleton, Interning
Problem: constructors in Java (and other OO languages) are inflexible
1. Can’t return a subtype of the type they belong to. “Factory” patterns address the issue: Factory method (e.g. createBicycle()), Factory class/object, Prototype
2. Always return a fresh new object, can’t reuse. “Sharing” patterns address the issue: Singleton, Interning
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

3. So Far: Structural Patterns
Wrappers: Adapter, Decorator, Proxy
Adapter: changes interface, preserves functionality
Decorator: preserves interface, changes functionality
Proxy: typically preserves both interface and functionality
Composite
A structural pattern: build complex whole-part structures, give uniform interface to client
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

4. So Far: Behavioral Patterns
Traversal of composites: Interpreter, Procedural and Visitor
Interpreter: groups code by object
Procedural, Visitor: group code by operation
Low (weak) coupling: Observer, Façade
Observer: decouples data (model) from views
Façade (actually, considered structural pattern): decouples client from complex library
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

5. Outline Behavioral patterns for traversing composites
Visitor, conclusion!
Behavioral pattern for traversal of containers
Iterator
Design exercise
More patterns: Strategy and Command, others
Odds and ends on creating objects

6. Iterator Pattern
Motivation: access the elements of a collection without exposing representation
AbstractList
Iterator
iterator() equals()
hasNext() next()
LinkedList
ArrayList
LinkedListIterator
ArrayListIterator
iterator()
iterator()
hasNext() next()
hasNext() next()
Spring 18 CSCI 2600, K. Kuzmin, A Milanova
creates

7. Iterator Pattern Visitor vs. Iterator?
Visitor pattern is similar to Iterator but different because it has knowledge of structure, not just sequence
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

8. Template Method Pattern
Motivation: factors out skeleton of algorithm, deferring some steps to subclasses
AbstractList
Iterator
iterator() equals()
hasNext() next()
Which one is the template method here?
Where else did we see template method? (Answer: createRace() in Bicycle Hierarchy.)
LinkedList
ArrayList
LinkedListIterator
ArrayListIterator
iterator()
iterator()
hasNext() next()
hasNext() next()
Spring 18 CSCI 2600, K. Kuzmin, A Milanova
creates

9. Design Patterns Summary so Far
Factory method, Factory object, Prototype
Creational patterns: address problem that constructors can’t return subtypes
Singleton, Interning
Creational patterns: address problem that constructors always return a new instance of class
Wrappers: Adapter, Decorator, Proxy
Structural patterns: when we want to change interface or functionality of an existing class, or restrict access to an object
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

10. Design Patterns Summary so Far
Composite
A structural pattern: expresses whole-part structures, gives uniform interface to client
Interpreter, Procedural, Visitor
Behavioral patterns: address the problem of how to traverse composite structures
Iterator
Template method

11. A Design Exercise
We are building a document editor --- a large rectangle that displays a document. Document can mix text, graphical shapes, etc. Surrounding the document are the menu, scrollbars, borders, etc.
Structure, Formatting, Embellishing the UI, User commands, Spell checking
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

12. Structure
Hierarchical structure --- document is made of columns, a column is made up of rows, a row is made up of words, images, etc.
Editor should treat text and graphics uniformly. Editor should treat simple and complex elements uniformly
What design pattern?
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

13. The Composite Pattern 1 Glyph Editor … draw() intersect() * Character
Polygon
Word
Row
draw() intersect()
draw() intersect()
draw() intersect()
draw() intersect() 1
What’s missing from this picture?
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

14. Formatting Formatting displays the document
Many different formatting strategies are possible
We would use different formatting strategies over the same hierarchical structure
Each formatting strategy is complex
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

15. The Strategy Pattern Encapsulates an algorithm in an object Glyph *
draw() …
FormattingStrategy …
compose() setDocument()
Document 1
draw() insert(…)
Strategy1
Strategy2
Strategy3
compose()
compose()
compose()
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

16. Embellishing the UI
We would like to embellish the document display. One embellishment adds a border around the document area, another one adds a horizontal scroll bar and a third one adds a vertical scroll bar
We would like to do this dynamically --- one can create any combination of embellished documents
What pattern?
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

17. The Decorator Pattern Adds functionality, preserves interface 1 Glyph*
draw() intersect()
component.draw();
Document
DecoratedGlyph 1
draw() intersect() insert() 1
draw()
component
BorderDecorator
ScrollDecorator
draw() drawBorder()
draw() drawScroll()
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

18. User Commands
Editor supports many user “commands”: open and close, cut and paste, etc.
There is different user interface for the same command
E.g., close document through a pull-down menu item, close button, key shortcut, other
Supports undo and redo!
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

19. A Naïve Design What’s wrong with this design? UIWidget draw() …
Document
MenuItem
ShortcutButton
draw() intersect() insert()
draw() …
draw() …
CloseItem
CloseButton … … … …
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

20. The Command Pattern Separates MenuItems from Commands that do the work
onClick() …
command
execute() 1 1
Document
command.execute();
Save
Close
open() save() close()
execute() …
execute() …
… = new MenuItem(“Save”, new Save(document));
… = new MenuItem(“Close”, new Close(document)); …
… = new ShortcutButton(“Save”, new Save(document));
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

21. Easy to Add Undo/Redo!
Editor maintains a history (e.g., a stack) of commands that have been executed
MenuItem
Command
command
onClick() …
execute() unexecute() 1 1
command.execute();
Cut
Paste
Document
execute() unexecute()
storedState
execute() unexecute()
storedState
open() save() close()
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

22. Adding Spell Checking Requires traversal of document hierarchy
We want to avoid writing this functionality into the document structure
We would like to add other traversals in the future, e.g., search, word count, hyphenation
What pattern?

23. The Visitor Pattern Glyph * * draw() intersect() accept(Visitor)
Document
Polygon
Word
draw() intersect() insert()
accept(Visitor)
draw() intersect()
accept(Visitor)
draw() intersect()
accept(Visitor) 1 1
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

24. Outline of Today’s Class
Behavioral patterns for traversing composites
Visitor, conclusion!
Behavioral pattern for traversal of containers
Iterator
Design exercise
More patterns: Strategy and Command, others
Odds and ends on creating objects
Static factory methods, and clone

25. Static Factory Methods
Not the same as GoF Factory Method/Object
Use static factory methods (typically) instead of, (sometimes) in addition to constructors
class Boolean { …
private Boolean(boolean b) { … }
public static Boolean valueOf(boolean b) { if (b) then return TRUE; else return FALSE; }
The examples I used so far conform with the GoF definition of Factory method: we have the parallel hierarchy of Objects and their Products and each Object class creates its own product.
The term “factory method” is used outside of this context. In general “Factory method” refers to any method that can return a Product (maybe a subtype), depending on the context.

26. Static Factory Methods in JDK
DateFormat class encapsulates knowledge on how to format a Date
Options: Just date? Just time? date+time? where in the world?
DateFormat df1 = DateFormat.getDateInstance();
DateFormat df2 = DateFormat.getTimeInstance();
Date today = new Date();
df1.format(today); // “Jul 4, 1776”
df2.format(today); // “10:15:00 AM”
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

27. Advantages
Does not need create a new object each time. Interning (use ==, save memory)
Can have descriptive names
Can hide classes
java.util.EnumSet can be instantiated only by static factory methods: e.g., of(E e1, E e2)
If less than 64 elements, creates a RegularEnumSet, otherwise creates a JumboEnumSet
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

28. Disadvantages Classes with public constructors cannot be subclassed
Look like regular static methods
newInstance, getInstance of
valueOf
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

29. Override clone() Judiciously
Remember Prototype creational pattern
class Race {
Bicycle bproto;
// constructor
Race(Bicycle bproto) {
this.bproto = bproto; }
Race createRace() {
Bicycle bike1 = bproto.clone(); …
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

30. Override clone() Judiciously
Object has protected Object clone()
Each class can define a clone method that returns a copy of the receiver object:
class Bicycle {
Bicycle clone() { return new Bicycle(); } }
So what’s the problem with clone()?
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

31. Override clone() Judiciously
To advertise that an object implements clone(), use the Cloneable interface:
class Bicycle implements Cloneable { … }
Invoking Object.clone() on an object that does not implement Cloneable throws exception
Object.clone() on an object that does implement Cloneable alters behavior of Object.clone(). Does a shallow copy. 1.

32. Cloning Point
class Point { int x, y; public Point(int x, int y) { this.x = x; this.y = y; } Point p1 = new Point(2,10); try { Point p2 = p1.clone(); } catch (CloneNotSupportedException e) { ... }
What happens? Compiles. At run-time throws CloneNotSupportedException.
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

33. Cloning Point again
class Point implements Cloneable { int x, y; public Point(int x, int y) { this.x = x; this.y = y; } Point p1 = new Point(2,10); try { Point p2 = p1.clone(); } catch (CloneNotSupportedException e) { ... }
What happens? Compiles, and runs fine. Creates a new Point object with copies of 2 and 10 fields. Since these are simple types, it’s all good.
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

34. Cloning Point again
class Point implements Cloneable { int x, y; public Point(int x, int y) { this.x = x; this.y = y; } public Point clone() { try { return (Point) super.clone(); } catch (CloneNotSupportedException e) {…}
What happens here? Compiles and runs just fine. Can clone points, and it is ok since these are just ints.
Using super.clone is the preferred way of cloning objects rather than calling a constructor. This is akin to constructors which initiate a chain of calls.
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

35. Cloning Stack TOTAL DISASTER! class Stack { private Object[] elements;
int size; …
public Stack clone() { try {
return (Stack) super.clone();
} catch (CloneNotSupportedException e) {…} }
TOTAL DISASTER! 1.
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

36. Cloning Stack better
public Stack clone() { try { Stack result = (Stack) super.clone(); result.elements = elements.clone(); return result; } catch (CloneNotSupportedException e) { throw new AssertionError(); } Yet, not always possible to clone fields recursively 1.
Spring 18 CSCI 2600, K. Kuzmin, A Milanova

37. Using clone(), recap
A class that implement Cloneable should override clone(). It should first call super.clone(), then replace all fields appropriately
Too complex!
Better not to use
Alternative: A copy constructor, or a static factory method analogous to a copy constructor 1.