List Iterators

    LinkedList<String> list = . . .;
    ListIterator<String> iterator = list.listIterator();
    while (iterator.hasNext())
    {
       String current = iterator.next();
       . . .
    }

• Why iterators?

Classical List Data Structure

• Traverse links directly
  

  Link currentLink = list.head;
  while (currentLink != null)
  {
     Object current = currentLink.data;
     currentLink = currentLink.next;
  }
  

• Exposes implementation
• Error-prone

List with Cursor

    for (list.reset(); list.hasNext(); list.next())
    {
        Object x = list.get();
        . . .
    }

• Disadvantage: Only one cursor per list
• Iterator is superior concept

The Pattern Concept

• History: Architectural Patterns
• Christopher Alexander
• Each pattern has
  • a short name
  • a brief description of the context
  • a lengthy description of the problem
  • a prescription for the solution

ITERATOR Pattern

Context

1. An aggregate object contains element objects
2. Clients need access to the element objects
3. The aggregate object should not expose its internal structure
4. Multiple clients may want independent access

Solution

1. Define an iterator that fetches one element at a time
2. Each iterator object keeps track of the position of the next element
3. If there are several aggregate/iterator variations, it is best if the aggregate and iterator classes realize common interface types.
   

Iterator Pattern

Iterator Pattern

• Names in pattern are examples
• Names differ in each occurrence of pattern

Name in Design Pattern	Actual Name (linked lists)
Aggregate	List
ConcreteAggregate	LinkedList
Iterator	ListIterator
ConcreteIterator	anonymous class implementing ListIterator
createIterator()	listIterator()
next()	next()
isDone()	opposite of hasNext()
currentItem()	return value of next()

Model/View/Controller

• Some programs have multiple editable views
• Example: HTML Editor
  • WYSIWYG view
    
  • structure view
  • source view
• Editing one view updates the other
• Updates seem instantaneous

Model/View/Controller

Model/View/Controller

• Model: data structure, no visual representation
• Views: visual representations
• Controllers: user interaction

• Views/controllers update model
• Model tells views that data has changed
• Views redraw themselves

Model/View/Controller

OBSERVER Pattern

• Model notifies views when something interesting happens
• Button notifies action listeners when something interesting happens
  
• Views attach themselves to model in order to be notified
• Action listeners attach themselves to button in order to be notified
• Generalize: Observers attach themselves to subject

Context


1. An object, called the subject, is source of events
2. One or more observer objects want to be notified when such an event occurs.

Solution

1. Define an observer interface type. All concrete observers implement it.
   
2. The subject maintains a collection of observers.
3. The subject supplies methods for attaching and detaching observers.
   
4. Whenever an event occurs, the subject notifies all observers.

Observer Pattern

Names in Observer Pattern

Name in Design Pattern	Actual Name (Swing buttons)
Subject	JButton
Observer	ActionListener
ConcreteObserver	the class that implements the ActionListener interface type
attach()	addActionListener()
notify()	actionPerformed()

Layout Managers

• User interfaces made up of components
• Components placed in containers
• Container needs to arrange components
• Swing doesn't use hard-coded pixel coordinates
• Advantages:
  • Can switch "look and feel"
  • Can internationalize strings
• Layout manager controls arrangement

Java Swing Layout Managers:

• FlowLayout: left to right, start new row when full
• BoxLayout: left to right or top to bottom
• BorderLayout: 5 areas, Center, North, South, East, West
• GridLayout: grid, all components have same size
• GridBagLayout: complex, like HTML table

Layout Managers

Layout Managers

• Set layout manager
  JPanel keyPanel = new JPanel();
  keyPanel.setLayout(new GridLayout(4, 3));
  
• Add components
  for (int i = 0; i < 12; i++)
     keyPanel.add(button[i]);

Layout Managers

Voice Mail System GUI

• Same backend as text-based system
  
• Only Telephone class changes
• Buttons for keypad
• Text areas for microphone, speaker

Voice Mail System GUI

• Arrange keys in panel with GridLayout:
  

  JPanel keyPanel = new JPanel(); 
  keyPanel.setLayout(new GridLayout(4, 3)); 
  for (int i = 0; i < 12; i++) 
  { 
     JButton keyButton = new JButton(...); 
     keyPanel.add(keyButton); 
     keyButton.addActionListener(...);
  }
  

• Panel with BorderLayout for speaker
  

  JPanel speakerPanel = new JPanel(); 
  speakerPanel.setLayout(new BorderLayout()); 
  speakerPanel.add(new JLabel("Speaker:"), BorderLayout.NORTH); 
  speakerField = new JTextArea(10, 25); 
  speakerPanel.add(speakerField, BorderLayout.CENTER);
  

Voice Mail System GUI

Containers, Components, and COMPOSITE Pattern

• Containers collect GUI components
• Sometimes, want to add a container to another container
• Container should be a component
• COMPOSITE design pattern
• Composite method typically invoke component methods
• E.g. Container.getPreferredSize invokes getPreferredSize of components

COMPOSITE Pattern

Context

1. Primitive objects can be combined to composite objects
2. Clients treat a composite object as a primitive object
   

Solution

1. Define an interface type that is an abstraction for the primitive objects
2. Composite object collects primitive objects
   
3. Composite and primitive classes implement same interface type.
4. When implementing a method from the interface type, the composite class applies the method to its primitive objects and combines the results

Composite Pattern

Composite Pattern

Name in Design Pattern	Actual Name (AWT components)
Primitive	Component
Composite	Container
Leaf	a component without children (e.g. JButton)
method()	a method of Component (e.g. getPreferredSize)

Scroll Bars

• Scroll bars can be attached to components
• Approach #1: Component class can turn on scroll bars
• Approach #2: Scroll bars can surround component
  

  JScrollPane pane = new JScrollPane(component);
  

• Swing uses approach #2
• JScrollPane is again a component

Scroll Bars

DECORATOR Pattern

Context

1. Component objects can be decorated (visually or behaviorally enhanced)
2. The decorated object can be used in the same way as the undecorated object
3. The component class does not want to take on the responsibility of the decoration
4. There may be an open-ended set of possible decorations

Solution

1. Define an interface type that is an abstraction for the component
2. Concrete component classes realize this interface type.
3. Decorator classes also realize this interface type.
4. A decorator object manages the component object that it decorates
5. When implementing a method from the component interface type, the decorator class applies the method to the decorated component and combines the result with the effect of the decoration.

Decorator Pattern

Decorator Pattern: Scroll Bars

Name in Design Pattern	Actual Name (scroll bars)
Component	Component
ConcreteComponent	JTextArea
Decorator	JScrollPane
method()	a method of Component (e.g. paint)

Streams

    InputStreamReader reader = new InputStreamReader(System.in);
    BufferedReader console = new BufferedReader(reader);

• BufferedReader takes a Reader and adds buffering
• Result is another Reader: Decorator pattern
• Many other decorators in stream library, e.g. PrintWriter
  

Decorator Pattern: Input Streams

Name in Design Pattern	Actual Name (input streams)
Component	Reader
ConcreteComponent	InputStreamReader
Decorator	BufferedReader
method()	read

How to Recognize Patterns

• Look at the intent of the pattern
• E.g. COMPOSITE has different intent than DECORATOR
  
•  Remember common uses (e.g. STRATEGY for layout managers)
• Not everything that is strategic is an example of STRATEGY pattern
  
• Use context and solution as "litmus test"

Litmus Test

• Can add border to Swing component
  

  Border b = new EtchedBorder()
  component.setBorder(b);
  

• Undeniably decorative
• Is it an example of DECORATOR?

Litmus Test

1. Component objects can be decorated (visually or behaviorally enhanced)
   PASS
   
2. The decorated object can be used in the same way as the undecorated object
   PASS
   
3. The component class does not want to take on the responsibility of the decoration
   FAIL--the component class has setBorder method
   
4. There may be an open-ended set of possible decorations

Putting Patterns to Work