Adapter pattern

In software engineering, the adapter pattern is a software design pattern (also known as wrapper, an alternative naming shared with the decorator pattern) that allows the interface of an existing class to be used as another interface. It is often used to make existing classes work with others without modifying their source code.

An 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.

Overview

The adapter design pattern is one of the twenty-three well-known Gang of Four design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

The adapter design pattern solves problems like:

• How can a class be reused that does not have an interface that a client requires?
• How can classes that have incompatible interfaces work together?
• How can an alternative interface be provided for a class?

Often an (already existing) class can not be reused only because its interface does not conform to the interface clients require.

The adapter design pattern describes how to solve such problems:

• Define a separate <code>adapter</code> class that converts the (incompatible) interface of a class (<code>adaptee</code>) into another interface (<code>target</code>) clients require.
• Work through an <code>adapter</code> to work with (reuse) classes that do not have the required interface.

The key idea in this pattern is to work through a separate <code>adapter</code> that adapts the interface of an (already existing) class without changing it.

Clients don't know whether they work with a <code>target</code> class directly or through an <code>adapter</code> with a class that does not have the <code>target</code> interface.

See also the UML class diagram below.

Definition

An adapter allows two incompatible interfaces to work together. This is the real-world definition for an adapter. Interfaces may be incompatible, but the inner functionality should suit the need. The adapter design pattern allows otherwise incompatible classes to work together by converting the interface of one class into an interface expected by the clients.

Usage

An adapter can be used when the wrapper must respect a particular interface and must support polymorphic behavior. Alternatively, a decorator pattern makes it possible to add or alter behavior of an interface at run-time, and a facade pattern is used when an easier or simpler interface to an underlying object is desired.

Structure

UML class diagram

In the above UML class diagram, the <code>client</code> class that requires a <code>target</code> interface cannot reuse the <code>adaptee</code> class directly because its interface doesn't conform to the <code>target</code> interface. Instead, the <code>client</code> works through an <code>adapter</code> class that implements the <code>target</code> interface in terms of <code>adaptee</code>:

• The <code>object adapter</code> way implements the <code>target</code> interface by delegating to an <code>adaptee</code> object at run-time (<code>adaptee.specificOperation()</code>).
• The <code>class adapter</code> way implements the <code>target</code> interface by inheriting from an <code>adaptee</code> class at compile-time (<code>specificOperation()</code>).

Object adapter pattern

In this 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.

Class adapter pattern

This adapter pattern uses multiple polymorphic interfaces implementing or inheriting both the interface that is expected and the interface that is pre-existing. It is typical for the expected interface to be created as a pure interface class, especially in languages such as Java (before JDK 1.8) that do not support multiple inheritance of classes.

A further form of runtime adapter pattern

Motivation from compile time solution

It is desired for to supply with some data, let us suppose some data. A compile time solution is:

However, suppose that the format of the string data must be varied. A compile time solution is to use inheritance:

and perhaps create the correctly "formatting" object at runtime by means of the factory pattern.

Run-time adapter solution

A solution using "adapters" proceeds as follows:

Implementation of the adapter pattern

When implementing the adapter pattern, for clarity, one can apply the class name to the provider implementation; for example, . It should have a constructor method with an adaptee class variable as a parameter. This parameter will be passed to an instance member of . When the clientMethod is called, it will have access to the adaptee instance that allows for accessing the required data of the adaptee and performing operations on that data that generates the desired output.

Java

Output <pre> Recharging android with MicroUsb MicroUsb connected Recharge started Recharge finished Recharging iPhone with Lightning Lightning connected Recharge started Recharge finished Recharging iPhone with MicroUsb MicroUsb connected Lightning connected Recharge started Recharge finished </pre>

Python

C#

Output:

See also

• Adapter Java Design Patterns - Adapter
• Delegation, strongly relevant to the object adapter pattern.
• Dependency inversion principle, which can be thought of as applying the adapter pattern, when the high-level class defines its own (adapter) interface to the low-level module (implemented by an adaptee class).
• Ports and adapters architecture
• Shim
• Wrapper function
• Wrapper library

References