• Defined:
  • "Descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context." (Gamma, Helm, Johnson and Vlissides)
• Level of Abstraction:
  • Design patterns are at a higher level of abstraction than data structures and algorithms

• Types:
  • Creational (i.e. concerned with object creation)
  • Structural (i.e., concerned with the composition of objects)
  • Behavioral (i.e., concerned with interactions and responsibilities)
• Benefits:
  • Simplifies the development of designs
  • Promotes better communication
  • May increase re-use and quality

• Convince you that you have something to learn
• Convince you that design patterns are useful
• Discuss some design patterns (that even beginning programmers can understand and use)

• Some applications require exactly one instance of a class:
  • Windowing systems with one event queue
  • Word processors with one menu bar (for all documents)
• Unfortunately, constructors can be called repeatedly:
  • In one method
  • In one class
  • Across multiple classes

A FileViewer

javaexamples/singleton/v1/FileViewer.java (Fragment: skeleton)

/**
 * A Frame that is used to view the contents of a file
 *
 * This class  uses the Singleton pattern 
 * to ensure that there is only one
 * instance at any point in time.
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 *
 */
public class FileViewer
{

    private static boolean      exists = false;
    private static FileViewer   instance;

    /**
     * Construct a new FileViewer
     *
     * Note: This method is private and used only internally
     */
    private FileViewer()
    {


        exists = true;
    }

}

        

javaexamples/singleton/v1/FileViewer.java (Fragment: createInstance)

    /**
     * Construct a new FileViewer (if one doesn't exist)
     * or return the existing instance
     */
    public static FileViewer createInstance()
    {
       if (!exists) instance = new FileViewer(); // exists will become true
       
       return instance;

    }
        

Using a FileViewer

javaexamples/singleton/v1/FileChooser.java (Fragment: valueChanged)

    /**
     * Handle valueChanged events 
     * (required by ListSelectionListener)
     *
     * @param lse   The ListSelectionEvent to handle
     */
    public void valueChanged(ListSelectionEvent lse)
    {
        FileViewer   fv;
        String       fn;

        fn = (String)list.getSelectedValue();

        fv = FileViewer.createInstance();
        fv.load(fn);
    }
        

• Computers vs. Calculators:
  • Computers can perform the same operation many times
• Programming Languages:
  • Harness this power using loops
• A Problem:
  • Traditional looping requires an understanding of the structure of the "aggregate"

Looping Over an Array

javaexamples/iterator/ArrayExample.java (Fragment: 1)

       String    city;

       for (int i=0; i < cities.length; i++)
       {
          city = cities[i];
          System.out.println(city);
       }
        

Looping Over a Vector

javaexamples/iterator/VectorExample.java (Fragment: 1)

       String    city;

       for (int i=0; i < cities.size(); i++)
       {
          city = (String)cities.elementAt(i);
          System.out.println(city);
       }
        

In UML:

• The Iterator:
  • Enumeration
• Some Aggregates:
  • Vector
  • Hashtable

• Many Objects:
  • Manage information/content
  • Present/display that information
  • React to events
• Examples:
  • Components/widgets in a GUI
  • Database management systems

Handling an arrowkeyPressed Event

Handling a mouseUp Event

Handling a characterKeyPressed Event

Creating an Integer Entry Field

javaexamples/mvc/IntegerDocument.java

package mvc;

import javax.swing.text.*;

/**
 * A Document that can only contain integer values
 *
 * Note: In order to be simple and clear, this implementation
 * does not support numbers with a leading '+' and will not allow
 * the document to contain only a '-'.
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class IntegerDocument extends PlainDocument
{

    /**
     * Check to see if an insertString() will result in an Integer or
     * not.
     *
     * @param  offset  The starting offset
     * @param  text    The string to insert
     * @return         true if OK; false otherwise
     */
    public boolean checkInsert(int offset, String text)
    {
       boolean      result;       
       String       newText;
       
       try 
       {
          newText = super.getText(0, offset) + text + 
             super.getText(offset, getLength()-offset);

          Integer.parseInt(newText);
          result = true;
       }
       catch (Exception e)
       {
          result = false;
       }

       return result;       
    }

    /**
     * Check to see if a remove() will result in an Integer or not.
     *
     * @param  offset  The starting offset
     * @param  length  The length to remove
     * @return         true if OK; false otherwise
     */
    public boolean checkRemove(int offset, int remove)
    {
       boolean      result;       
       String       newText;
       
       try 
       {
          newText = super.getText(0, offset) + 
             super.getText(offset+remove, getLength()-offset-remove);

          if (!newText.equals("")) Integer.parseInt(newText);
          result = true;
       }
       catch (Exception e)
       {
          result = false;
       }

       return result;       
    }
    

    /**
     * Check to see if a replace() will result in an Integer or
     * not.
     *
     * @param  offset  The starting offset
     * @param  length  The length to remove
     * @param  text    The string to insert
     * @return         true if OK; false otherwise
     */
    public boolean checkReplace(int offset, int remove, String text)
    {
       boolean      result;       
       String       newText;
       
       try 
       {
          newText = super.getText(0, offset) + text + 
             super.getText(offset+remove, getLength()-offset-remove);

          if (!newText.equals("")) Integer.parseInt(newText);
          result = true;
       }
       catch (Exception e)
       {
          result = false;
       }

       return result;       
    }
    

    /**
     * Inserts content into the document (but only if 
     * the result is an Integer)
     *
     * @param offset   The starting offset
     * @param text     The string to insert
     * @param a        The attributes for the inserted text
     */
    public void insertString(int offset, String text, AttributeSet a) 
                throws BadLocationException
    {
       if (checkInsert(offset, text)) super.insertString(offset, text, a);
    }

    /*
     * Remove some content from the document (but only if the result is
     * a SimpleTime).
     *
     * @param offset The starting offset
     * @param length The number of characters to remove 
     */
    public void remove(int offset, int length) 
                throws BadLocationException
    {
       if (checkRemove(offset, length)) super.remove(offset, length);
    }


    /**
     * Deletes the region of text from offset to offset + length, and
     * replaces it with text.
     *
     * @param offset The starting offset
     * @param length The number of characters to remove 
     * @param text   The string to insert
     * @param a      The attributes for the inserted text
     */
    public void replace(int offset, int length, String text, AttributeSet a)  
                throws BadLocationException
    {
       if (checkReplace(offset, length, text)) 
          super.replace(offset, length, text, a);
    }
}
        

javaexamples/mvc/IntegerEntryField.java

package mvc;

import javax.swing.*;
import javax.swing.text.*;

/**
 * An entry field that only accepts integers
 *
 * Note: In order to be simple and clear, this implementation
 * does not support numbers with a leading '+' and will not allow
 * the document to contain only a '-'.
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0 
 */
public class IntegerEntryField extends JTextField
{
    /**
     * Create a Document (i.e., model) for this IntegerEntryField
     *
     * @return  An IntegerDocument
     */
    protected Document createDefaultModel()
    {
        return new IntegerDocument();
    }
}
        

Creating a Password Entry Field

What would you change and why?

Skins

What would you change and why?

• Some Examples:
  • Command Pattern
  • Composite Pattern
  • Decorator Pattern
  • Facade Pattern
  • Observer Pattern
  • Proxy Pattern
• Prerequesites - You Must Understand:
  • Specialization and Inheritance
  • Interfaces
  • Abstract Classes

• Adding Capabilities to a Class:
  • Specialization
• Adding Capabilities to an Object:
  • Needs to be done "dynamically" (i.e., at run time)

javaexamples/decorator/Printer.java

/**
 * The requirements of a Printer
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public interface Printer
{
    /**
     * Print the given String
     *
     * @param text   The String to print
     */
    public abstract void print(String text);    
}

        

javaexamples/decorator/ConsolePrinter.java

/**
 * Prints text to the console
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class ConsolePrinter implements Printer
{
    /**
     * Print the given String
     *
     * @param text   The String to print
     */
    public void print(String text)
    {
       System.out.print(text);       
    }
    

}
        

javaexamples/decorator/PrinterDecorator.java

/**
 * An abstract decorator of Printer
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public abstract class PrinterDecorator implements Printer
{
    protected Printer     decorated;
    
    /**
     * Explicit Value Constructor
     *
     * @param decorated    The Printer to decorate
     */
    public PrinterDecorator(Printer decorated)
    {
       this.decorated = decorated;       
    }
    

    /**
     * Print the given String
     * (required by Printer)
     *
     * @param text   The String to print
     */
    public void print(String text)
    {
        decorated.print(text);
    }
}
        

javaexamples/decorator/UppercasePrinter.java

/**
 * A Printer that always prints in uppercase
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class UppercasePrinter extends PrinterDecorator
{
    /**
     * Explicit Value Constructor
     *
     * @param decorated    The Printer to decorate
     */
    public UppercasePrinter(Printer decorated)
    {
       super(decorated);       
    }
    

    /**
     * Print the given String
     * (required by Printer)
     *
     * @param text   The String to print
     */
    public void print(String text)
    {
       decorated.print(text.toUpperCase());       
    }
}

        

javaexamples/decorator/WrappingPrinter.java

import java.util.*;

/**
 * A Printer that wraps at word boundaries
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class WrappingPrinter extends PrinterDecorator
{
    protected int         width;
    

    /**
     * Explicit Value Constructor
     *
     * @param decorated    The Printer to decorate
     * @param width        The maximum width
     */
    public WrappingPrinter(Printer decorated, int width)
    {
       super(decorated);       
       this.width = width;       
    }
    

    /**
     * Print the given String
     * (required by Printer)
     *
     * @param text   The String to print
     */
    public void print(String text)
    {
       int                    required, used;       
       String                 token;       
       StringTokenizer        st;
       
       st   = new StringTokenizer(text);
       used = 0;       

       while (st.hasMoreTokens())
       {
          token    = st.nextToken();
          required = token.length();
          
          if ((required + used + 1) > width)
          {
             decorated.print("\n");
             decorated.print(token);             
             used = required + 1;
          }
          else
          {
             if (used != 0) 
             {
                decorated.print(" ");
                ++used;
             }
             
             decorated.print(token);
             used += required;             
          }
       }
    }
}

        

javaexamples/decorator/Driver.java

/**
 * An application that demonstrates the use of the
 * Decorator Pattern
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class Driver
{
    /**
     * The entry point
     *
     * @param args   The command line arguments
     */
    public static void main(String[] args)
    {
       Printer           printer;       
       String            text;
       

       text = "This is the text that we will use " +
              "to demonstrate the capabilities "   +
              "of different Printer objects.";
       

       printer = new ConsolePrinter();
       printer.print(text);       

       System.out.print("\n\n");       

       printer = new UppercasePrinter(new ConsolePrinter());
       printer.print(text);       

       System.out.print("\n\n");       

       printer = new WrappingPrinter(new ConsolePrinter(), 20);
       printer.print(text);       

       System.out.print("\n\n");       

       printer = new WrappingPrinter(
                     new UppercasePrinter(
                         new ConsolePrinter()), 20);
       printer.print(text);       
    }
    
}