Two Design Patterns: Iterator and Adapter

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Two Design PatternsIterator and Adapter

• C Sc 335
• Rick Mercer

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Iterator

• Name Iterator (a.k.a Enumeration)
• Problem How can you traverse over all objects in
  any collection. You dont want to change client
  code when the collection changes. You also want
  the same interface (methods)
• Solutions
• Design an interface
• Have each class implement an interface.

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Java 1.4 version of Iterator

• public interface Iterator
• // Return true if the iteration has at least
• // one more elements to visit
• boolean hasNext()
• // Returns the next element in the iteration
• Object next()
• // Removes from the underlying collection the
  last
• // element returned by the iterator (optional)
  void remove()

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Java 5 version of Iterator

• public interface IteratorltEgt
• // Return true if the iteration has at least
• // one more elements to visit
• boolean hasNext()
• // Returns the next element in the iteration
• E next()
• // Removes from the underlying collection the
  last
• // element returned by the iterator (optional)
  void remove()

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Java 1.4 Code

• Map h new TreeMap()
• h.put("one", "1 value")
• h.put("two", "vaLue 2!")
• h.put("three", "Value Three")
• h.put("four", "Value_4th")
• Collection c h.values()
• Iterator itr c.iterator()
• String str null
• while (itr.hasNext())
• str (String) itr.next() // Cast required
• System.out.println(str.toUpperCase())
• Set set h.keySet()
• System.out.println(set)

VALUE_4TH 1 VALUE VALUE THREE VALUE 2! four,
one, three, two
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Java 1.4 Code

• MapltString, Stringgt h new TreeMapltString,
  Stringgt()
• h.put("one", "1 value")
• h.put("two", "vaLue 2!")
• h.put("three", "Value Three")
• h.put("four", "Value_4th")
• CollectionltStringgt c h.values()
• IteratorltStringgt itr c.iterator()
• String str null
• while (itr.hasNext())
• // no cast, Java does the cast for us unboxs
• System.out.println(itr.next().toUpperCase())
• Set set h.keySet()
• System.out.println(set)

VALUE_4TH 1 VALUE VALUE THREE VALUE 2! four,
one, three, two
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Benefits

• Consequences
• Can change collection class without changing code
  to traverse the collection
• Can provide a well-known interface to other
  objects
• Can iterate over collections that have different
  data structures to store elements
• Let the iterator do the work

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UML Diagram of Java 1.4's Iterator and List
interface
ltltinterfacegtgt List iterator()
ltltinterfacegtgt Iterator hasNext() next()
Vector iterator()
LinkedList iterator()
ArrayList iterator()
Iterator hasNext() next()
Context
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Adapter

• The intent of Adapter is to provide the interface
  a client expects, using the services of a class
  with a different interface
• You may have used Adapter (also known as Wrapper)
• Adapted HashMap to Jukebox to allow only 3
  methods particular to the Jukebox
• Provides type safety can only add Song objects
• Avoids a lot of casts have the get method return
  Song objects, not Object objects

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• Adapted ArrayList to store Songs
• Provides type safety can only add Song objects
• Avoids a lot of casts have the get method return
  Song objects, not Object objects
• Not as compelling a usage in Java 5
• generics provides type safety
• remove the need to cast

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Object Adapter

• Sometime the Adapter extends a class or
  implements an interface used in the client code
• JTable needs an instance of TableModel that
  represents the model (data)
• Your new class must have methods such as
• getColumnCount, getRowCount, getValueAt
• Why? Because JTable uses them to populate view
• You adapt your data to the interface expected by
  JTable by implementing all 10 methods

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Show Table of Employees

• Employee w1
• new Employee("Devon", 40, 15.75, 3, "M")
• Employee w2
• new Employee("Kim", 0, 12.50, 1, "S")
• Employee w3
• new Employee("Chris", 35, 20.50, 2, "M")
• Employee employees w1, w2, w3
• EmployeeTable empTable
• new EmployeeTable(employees)

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EmployeeTable class

• // Store an array of Employee objects that
• // can be put into a table
• class EmployeeTable implements TableModel
• private int rows, cols
• private Employee data
• public EmployeeTable(Employee employeeData)
• data employeeData
• cols 4
• rows data.length
• // continued

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Team Activity

• public int getColumnCount() // TableModel
  interface
• // What goes here?
• public int getRowCount() // TableModel
  interface
• // What goes here?

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JTable Populates Cells

• // TableModel interface
• public Object getValueAt(int row, int col)
• if (col 0)
• return datarow.getName()
• else if (col 1)
• return "" datarow.grossPay()
• else if (col 2)
• return "" datarow.getTotalTaxes()
• else if (col 3)
• return "" (datarow.grossPay() -
• datarow.getTotalTaxes())
• else return null

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The View

• class EmployeeFrame extends JFrame
• private JTable view
• public EmployeeFrame(EmployeeTable empTable)
• setTitle("Adapter")
• setSize(300, 200)
• setDefaultCloseOperation(EXIT_ON_CLOSE)
• view new JTable(empTable)
• getContentPane().add(view)
• pack()

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Needs formatting
// Tell the JFrame what to show EmployeeFrame
window new EmployeeFrame(empTable) window.setVi
sible(true)