Adapter pattern

In computer programming, the adapter design pattern (often referred to as the wrapper pattern or simply a wrapper) 'adapts' one interface for a class into one that a client expects. An adapter allows classes to work together that normally could not because of incompatible interfaces by wrapping its own interface around that of an already existing class. The adapter is also responsible for handling any logic necessary to transform data into a form that is useful for the consumer. For instance, if multiple boolean values are stored as a single integer but your consumer requires a 'true'/'false', the adapter would be responsible for extracting the appropriate values from the integer value.

Structure

There are two types of adapter patterns:

Object Adapter pattern: In this type of adapter pattern, the adapter contains an instance of the class it wraps. In this situation, the adapter makes calls to the instance of the wrapped object.

The object adapter pattern expressed in UML. The adapter *hides* the adaptee's interface from the client.

Class Adapter pattern: This type of adapter uses multiple inheritance to achieve its goal. The adapter is created inheriting interfaces from both the interface that is expected and the interface that is pre-existing. Note that when the adapter must adapt multiple adaptees, the Class Adapter pattern cannot be used in languages such as Java that do not support multiple inheritance.

The class adapter pattern expressed in UML.

The adapter pattern is useful in situations where an already existing class provides some or all of the services you need but does not use the interface you need. A good real life example is an adapter that converts the interface of a Document Object Model of an XML document into a tree structure that can be displayed. A link to a tutorial that uses the adapter design pattern is listed in the links below.

Sample - Object Adapter

# Python code sample
class Target(object):
    def specific_request(self):
        return 'Hello Adapter Pattern!'

class Adapter(object):
    def __init__(self, adaptee):
        self.adaptee = adaptee
    
    def request(self):
        return self.adaptee.specific_request()

client = Adapter(Target())
print client.request()

Sample - Class Adapter

/**
 * Java code sample 
 */

/* the client class should instantiate adapter objects */
/* by using a reference of this type */ 
interface Stack<T>
{
  void push (T o);
  T pop ();
  T top ();
}

/* DoubleLinkedList is the adaptee class */
class DList<T>
{
  public void insert (DNode pos, T o) { ... }
  public void remove (DNode pos) { ... }
  
  public void insertHead (T o) { ... }
  public void insertTail (T o) { ... }
  
  public T removeHead () { ... }
  public T removeTail () { ... }
  
  public T getHead () { ... }
  public T getTail () { ... }
}

/* Adapt DList class to Stack interface is the adapter class */
class DListImpStack<T> extends DList<T> implements Stack<T>
{
  public void push (T o) {
    insertTail (o);
  }
  
  public T pop () {
    return removeTail ();
  }
  
  public T top () {
    return getTail ();
  }
}

External links

Types of Adapters with C# example
Adapter in UML and in LePUS3 (a formal modelling language)
Description in Portland Pattern Repository's Wiki
Java Tutorial on the Document Object Model (uses the adapter pattern)
Citations from CiteSeer
Jt J2EE Pattern Oriented Framework
Adapter Design Pattern in C# and VB.NET