Design Patterns

1
Design Patterns

• Section 7.1 (JIAs)
• Section 7.2.1 (till page 259) (JIAs)
• Section 7.2.2(JIAs)
• Section 10.4.1 (JIAs)

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Template Design Pattern

• What is a design pattern?
• Useful?
• Singleton?
• Template?

3
Adapter Design Pattern

• Hook up your laptop in Europe
• Pattern Name Adapter
• Category Structural design pattern
• Intent To convert the interface of a class into
  another interface that new clients expect
• The Adapter pattern is used so that two unrelated
  interfaces can work together
• The join between them is called an Adapter
• We convert interface of one class into interface
  expected by the client
• A.K.A Wrapper
• Applicability when we need to use an existing
  class with a different interface than desired

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Structure of Object Adapter Pattern
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Structure of Object Adapter Pattern

• Object adapters use delegation to adapt one
  interface to another
• The adapter implements the target interface that
  the client expects to see, while it holds an
  instance of the adaptee
• Can extend adaptee instead (limited to a single
  adaptee)
• Object (composition) vs. Class (inheritance)
  Adapter
• When the client calls the request() method on its
  target object (the adapter), the request is
  translated into the corresponding specific
  request on the adaptee
• Adapters enable the client and the adaptee to be
  completely decoupled from each other
• Only the adapter knows about both of them

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Structure of Class Adapter Pattern
Adaptee
7
Adapter Design Pattern

• Participants
• Client
• Requires objects conforming to ATarget
• ATarget interface
• Defines the interface expected by the Client
• Adaptee class
• Defines the undesired interface of an existing
  class
• Adapter
• Adapts the interface of Adaptee to ATarget via
  delegation (composition) or inhertince

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Example Object Adapter

• CLIENT application would like to use a stack
  interface on DList (i.e. adapt ADAPTEE to TARGET)
• ADAPTEE
• / DoubleLinkedList /
• class DList
• public void insert (DNode pos, Object o) ...
  
• public void remove (DNode pos, Object o) ...
  
• public void insertHead (Object o) ...
• public void insertTail (Object o) ...
• public Object removeHead () ...
• public Object removeTail () ...
• public Object getHead () ...
• public Object getTail () ...

• TARGET
• interface Stack
• void push (Object)
• Object pop ()
• Object top ()
• ADAPTER
• / Adapt DList class to Stack interface /
• class DListStack implements Stack
• private DList _dlist
• public DListStack()
• _dlist new DList()
• public void push (Object o) _dlist.insertTail
  (o)
• public Object pop ()
• return _dlist.removeTail ()
• public Object top ()
• return _dlist.getTail ()

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Example Class Adaptor

• / Java code sample /
• TARGET
• interface Stack
• void push (Object)
• Object pop ()
• Object top ()
• ADAPTEE
• /DoubleLinkedList /
• class DList
• public void insert (DNode pos, Object o) ...
• public void remove (DNode pos, Object o) ...
• public void insertHead (Object o) ...
• public void insertTail (Object o) ...
• public Object removeHead () ...
• public Object removeTail () ...
• public Object getHead () ...
• public Object getTail () ...

• / Adapt DList class to Stack interface /
• ADAPTER
• class DListImpStack extends DList implements
  Stack
• public void push (Object o) insertTail (o)
• public Object pop ()
• return removeTail ()
• public Object top ()
• return getTail ()
• Exam Question Given a Target and an Adaptee,
  devise an object or class adapter

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Example (Object Adapter)

• Example page 514-530
• Table (Generic display table)
• Client
• Expects entries of type TableEntry
• ListTableModel
• TableEntry (row in a table)
• Target
• Student (Information on a Student)
• Adaptee
• Does not conform to TableEntry
• StudentEntry2
• Adapter
• Adapts Student to TableEntry

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Example
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Example (Class Adapter)

• Table (Generic display table)
• Client
• Expects entries of type TableEntry
• TableEntry (row in a table)
• Target
• Student (Information on a Student)
• Adaptee
• Does not conform to TableEntry
• StudentEntry
• Adapter
• Adapts Student to TableEntry

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Example