Object-Oriented Design & Patterns

Cay S. Horstmann

Chapter 5

Patterns and GUI Programming

Chapter Topics

• Iterators
• The Pattern Concept
• The OBSERVER Pattern
• Layout Managers and the STRATEGY Pattern
• Components, Containers, and the COMPOSITE Pattern
• Scroll Bars and the DECORATOR Pattern
  
• How to Recognize Patterns
• Putting Patterns to Work
  

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

High-Level View of Data Structures

• Queue
  
  
  
  
• Array with random access
  
  
  
  
• List
  ???

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

Short Passages Pattern

Short Passages Pattern

Context

Problem

a lengthy description of the problem, including

• a depressing picture
• issues of light and furniture
• research about patient anxiety in hospitals
• research that suggests that corridors over 50 ft are considered uncomfortable

Short Passages Pattern

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

Iterator Pattern

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

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

Model/View/Controller

• 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

Observer Pattern

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

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

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

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(...);
  }
  

Voice Mail System GUI

• 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

• Add speaker, keypads, and microphone panel to frame
• Frame already has BorderLayout
  
• Ch5/mailgui/Telephone.java

Custom Layout Manager

• Form layout
• Odd-numbered components right aligned
• Even-numbered components left aligned
• Implement LayoutManager interface type
  

The LayoutManager Interface Type

public interface LayoutManager 
{ 
   void layoutContainer(Container parent); 
   Dimension minimumLayoutSize(Container parent); 
   Dimension preferredLayoutSize(Container parent); 
   void addLayoutComponent(String name, Component comp); 
   void removeLayoutComponent(Component comp); 
} 

Form Layout

• Ch5/layout/FormLayout.java
• Ch5/layout/FormLayoutTester.java
• Note: Can use GridBagLayout to achieve the same effect
  

Strategy Pattern

• Pluggable strategy for layout management
• Layout manager object responsible for executing concrete strategy
• Generalizes to Strategy Design Pattern
• Other manifestation: Comparators
  

  Comparator<Country> comp = new CountryComparatorByName();
  Collections.sort(countries, comp);
  

Strategy Pattern

Context

1. A class can benefit from different variants for an algorithm
2. Clients sometimes want to replace standard algorithms with custom versions

Solution

1. Define an interface type that is an abstraction for the algorithm
2. Actual strategy classes realize this interface type.
3. Clients can supply strategy objects
4. Whenever the algorithm needs to be executed, the context class calls the appropriate methods of the strategy object
   

Strategy Pattern

Strategy Pattern: Layout Management     

Strategy Pattern: Sorting

Containers and Components

• 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

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

Decorator Pattern

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

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

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

• Invoice contains line items
• Line item has description, price
• Interface type LineItem:
  Ch5/invoice/LineItem.java
  
• Product is a concrete class that implements this interface:
  Ch5/invoice/Product.java
  

Bundles

• Bundle = set of related items with description+price
• E.g. stereo system with tuner, amplifier, CD player + speakers
• A bundle has line items
• A bundle is a line item
• COMPOSITE pattern
• Ch5/invoice/Bundle.java (look at getPrice)
  

Bundles

Discounted Items

• Store may give discount for an item
• Discounted item is again an item
• DECORATOR pattern
• Ch5/invoice/DiscountedItem.java (look at getPrice)
• Alternative design: add discount to LineItem

Discounted Items

Model/View Separation

• GUI has commands to add items to invoice
• GUI displays invoice
• Decouple input from display
• Display wants to know when invoice is modified
• Display doesn't care which command modified invoice
• OBSERVER pattern

Change Listeners

• Use standard ChangeListener interface type
  public interface ChangeListener
  {
     void stateChanged(ChangeEvent event);
  }
  
• Invoice collects ArrayList of change listeners
• When the invoice changes, it notifies all listeners:
  ChangeEvent event = new ChangeEvent(this);
  for (ChangeListener listener : listeners)
     listener.stateChanged(event);
  

Change Listeners

• Display adds itself as a change listener to the invoice
• Display updates itself when invoice object changes state
  

  final Invoice invoice = new Invoice();
  final JTextArea textArea = new JTextArea(20, 40);
  ChangeListener listener = new
     ChangeListener()
     {
        public void stateChanged(ChangeEvent event)
        {
           textArea.setText(...);
        }
     };
  

Observing the Invoice

Iterating Through Invoice Items

• Invoice collect line items
• Clients need to iterate over line items
• Don't want to expose ArrayList
• May change (e.g. if storing invoices in database)
• ITERATOR pattern

Iterators

• Use standard Iterator interface type
  public interface Iterator<LineItem>
  {
     boolean hasNext();
     LineItem next();
     void remove();
  }
• remove is "optional operation" (see ch. 8)
• implement to throw UnsupportedException
• implement hasNext/next manually to show inner workings
• Ch5/invoice/Invoice.java

Iterators

Formatting Invoices

• Simple format: dump into text area
• May not be good enough,
• E.g. HTML tags for display in browser
• Want to allow for multiple formatting algorithms
• STRATEGY pattern

Formatting Invoices

• ch5/invoice/InvoiceFormatter.java
• ch5/invoice/SimpleFormatter.java
• ch5/invoice/InvoiceTester.java
  

Formatting Invoices

Formatting Invoices