Adapter pattern

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In computer programming, the adapter design pattern (sometimes 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.

There are two types of adapter patterns:

The 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 a physical instance of the wrapped object.

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

The 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. The Object Adapter pattern is more often used as some popular languages, such as Java, do not support true multiple inheritance as the designers of these languages consider it a dangerous practice.

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 - Class Adaptor1

/**
* Java code sample
*/

interface Stack


/* DoubleLinkedList */
class DList

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 */
class DListImpStack extends DList implements Stack


public Object pop () 


public Object top () 
}

Sample - Object Adapter

/**
* Java code sample
*/

interface Stack


/* DoubleLinkedList */
class DList

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 */
class DListStack implements Stack

public void push (Object o) 


public Object pop () 


public Object top () 
}

External links

[Description in Portland Pattern Repository's Wiki]
[Java Tutorial on the Document Object Model] (uses the adapter pattern)
[Citations from CiteSeer]

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