View source for Adapter pattern

{{Short description|Design pattern in computer programming}}
{{excessive examples|date=January 2011}}

In [[software engineering]], the '''adapter pattern''' is a [[software design pattern]] (also known as [[Wrapper function|wrapper]], an alternative naming shared with the [[decorator pattern]]) that allows the [[interface (computer science)|interface]] of an existing [[class (computer science)|class]] to be used as another interface.<ref name="HeadFirst"/>{{rp|244}} 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<ref name="GoF"/> design pattern is one of the twenty-three well-known [[Gang of Four (software)|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:<ref name="Adapter_Design_Pattern"/>
* 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 [[Polymorphism (computer science)|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.<ref name="HeadFirst"/>{{rp|243, 252, 258, 260}}
{| class="wikitable"
! Pattern !! Intent
|-
| Adapter or wrapper || Converts one interface to another so that it matches what the client is expecting
|-
| [[Decorator pattern|Decorator]] || Dynamically adds responsibility to the interface by wrapping the original code
|-
| [[Delegation pattern|Delegation]] || Support "composition over inheritance"
|-
| [[Facade pattern|Facade]] || Provides a simplified interface
|}

==Structure==
=== UML class diagram ===
[[File:w3sDesign Adapter Design Pattern UML.jpg|frame|none|A sample UML class diagram for the adapter design pattern.<ref name="Structure"/>]]

In the above [[Unified Modeling Language|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 (computing)|object]].

[[File:ObjectAdapter.png|thumb|300px|center|The object adapter pattern expressed in [[Unified Modeling Language|UML]]]]
[[File:Adapter(Object) pattern in LePUS3.png|thumb|500px|center|The object adapter pattern expressed in [[Lepus3|LePUS3]]]]

===Class adapter pattern===
This adapter pattern uses multiple [[Subtyping|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 (Java)|interface]] class, especially in [[programming language|languages]] such as [[Java (programming language)|Java]] (before JDK 1.8) that do not support [[multiple inheritance]] of classes.<ref name="HeadFirst"/>{{rp|244}}

[[File:ClassAdapter.png|thumb|300px|center|The class adapter pattern expressed in [[Unified Modeling Language|UML]].]]
[[File:Adapter(Class) pattern in LePUS3.png|thumb|500px|center|The class adapter pattern expressed in [[Lepus3|LePUS3]]]]

===A further form of runtime adapter pattern===
====Motivation from compile time solution====
It is desired for {{Java|classA}} to supply {{Java|classB}} with some data, let us suppose some {{Java|String}} data. A compile time solution is:

<syntaxhighlight lang=Java>
classB.setStringData(classA.getStringData());
</syntaxhighlight>

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

<syntaxhighlight lang=Java>
public class Format1ClassA extends ClassA {
    @Override
    public String getStringData() {
        return format(toString());
    }
}
</syntaxhighlight>

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:
{{ordered list|list-style-type=lower-roman
|Define an intermediary "provider" interface, and write an implementation of that provider interface that wraps the source of the data, {{Java|ClassA}} in this example, and outputs the data formatted as appropriate:

<syntaxhighlight lang=Java>
interface StringProvider {
    public String getStringData();
}

public class ClassAFormat1 implements StringProvider {
    private ClassA classA = null;

    public ClassAFormat1(final ClassA a) {
        classA = a;
    }

    public String getStringData() {
        return format(classA.getStringData());
    }

    private String format(final String sourceValue) {
        // Manipulate the source string into a format required 
        // by the object needing the source object's data
        return sourceValue.trim();
    }
}
</syntaxhighlight>

|Write an adapter class that returns the specific implementation of the provider:

<syntaxhighlight lang=Java>
public class ClassAFormat1Adapter extends Adapter {
    public Object adapt(final Object anObject) {
        return new ClassAFormat1((ClassA) anObject);
    }
}
</syntaxhighlight>

|Register the {{Java|adapter}} with a global registry, so that the {{Java|adapter}} can be looked up at runtime:

<syntaxhighlight lang=Java>
AdapterFactory.getInstance().registerAdapter(ClassA.class, ClassAFormat1Adapter.class, "format1");
</syntaxhighlight>

|In code, when wishing to transfer data from {{Java|ClassA}} to {{Java|ClassB}}, write:

<syntaxhighlight lang=Java>
Adapter adapter = AdapterFactory.getInstance()
    .getAdapterFromTo(ClassA.class, StringProvider.class, "format1");
StringProvider provider = (StringProvider) adapter.adapt(classA);
String string = provider.getStringData();
classB.setStringData(string);
</syntaxhighlight>

or more concisely:

<syntaxhighlight lang=Java>
classB.setStringData(((StringProvider)AdapterFactory.getInstance()
    .getAdapterFromTo(ClassA.class, StringProvider.class, "format1")
    .adapt(classA))
    .getStringData()
);
</syntaxhighlight>

|The advantage can be seen in that, if it is desired to transfer the data in a second format, then look up the different adapter/provider:

<syntaxhighlight lang=Java>
Adapter adapter = AdapterFactory.getInstance()
    .getAdapterFromTo(ClassA.class, StringProvider.class, "format2");
</syntaxhighlight>

|And if it is desired to output the data from {{Java|ClassA}} as, say, image data in {{Java|Class C}}:
<syntaxhighlight lang=Java>
Adapter adapter = AdapterFactory.getInstance()
    .getAdapterFromTo(ClassA.class, ImageProvider.class, "format2");
ImageProvider provider = (ImageProvider) adapter.adapt(classA);
classC.setImage(provider.getImage());
</syntaxhighlight>

|In this way, the use of adapters and providers allows multiple "views" by {{Java|ClassB}} and {{Java|ClassC}} into {{Java|ClassA}} without having to alter the class hierarchy. In general, it permits a mechanism for arbitrary data flows between objects that can be retrofitted to an existing object hierarchy.
}}
<!-- Wikipedia is not a list of examples. Do not add examples from your favorite programming language here; this page exists to explain the design pattern, not to show how it interacts with subtleties of every language under the sun. Feel free to add examples here: http://en.wikibooks.org/wiki/Computer_Science/Design_Patterns/Adapter -->

===Implementation of the adapter pattern===
When implementing the adapter pattern, for clarity, one can apply the class name {{Java|[ClassName]To[Interface]Adapter}} to the provider implementation; for example, {{Java|DAOToProviderAdapter}}. It should have a constructor method with an adaptee class variable as a parameter. This parameter will be passed to an instance member of {{Java|[ClassName]To[Interface]Adapter}}. 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====
<syntaxhighlight lang=Java>
package org.wikipedia.examples;

interface ILightningPhone {
    void recharge();
    void useLightning();
}

interface IMicroUsbPhone {
    void recharge();
    void useMicroUsb();
}

class Iphone implements ILightningPhone {
    private boolean connector;

    @Override
    public void useLightning() {
        connector = true;
        System.out.println("Lightning connected");
    }

    @Override
    public void recharge() {
        if (connector) {
            System.out.println("Recharge started");
            System.out.println("Recharge finished");
        } else {
            System.out.println("Connect Lightning first");
        }
    }
}

class Android implements IMicroUsbPhone {
    private boolean connector;

    @Override
    public void useMicroUsb() {
        connector = true;
        System.out.println("MicroUsb connected");
    }

    @Override
    public void recharge() {
        if (connector) {
            System.out.println("Recharge started");
            System.out.println("Recharge finished");
        } else {
            System.out.println("Connect MicroUsb first");
        }
    }
}
/* exposing the target interface while wrapping source object */
class LightningToMicroUsbAdapter implements IMicroUsbPhone {
    private final ILightningPhone lightningPhone;

    public LightningToMicroUsbAdapter(ILightningPhone lightningPhone) {
        this.lightningPhone = lightningPhone;
    }

    @Override
    public void useMicroUsb() {
        System.out.println("MicroUsb connected");
        lightningPhone.useLightning();
    }

    @Override
    public void recharge() {
        lightningPhone.recharge();
    }
}

public class AdapterDemo {
    static void rechargeMicroUsbPhone(IMicroUsbPhone phone) {
        phone.useMicroUsb();
        phone.recharge();
    }

    static void rechargeLightningPhone(ILightningPhone phone) {
        phone.useLightning();
        phone.recharge();
    }

    public static void main(String[] args) {
        Android android = new Android();
        Iphone iPhone = new Iphone();

        System.out.println("Recharging android with MicroUsb");
        rechargeMicroUsbPhone(android);

        System.out.println("Recharging iPhone with Lightning");
        rechargeLightningPhone(iPhone);

        System.out.println("Recharging iPhone with MicroUsb");
        rechargeMicroUsbPhone(new LightningToMicroUsbAdapter(iPhone));
    }
}
</syntaxhighlight>

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====
<syntaxhighlight lang="python">
"""
Adapter pattern example.
"""
from abc import ABCMeta, abstractmethod
from typing import NoReturn

RECHARGE: list[str] = ["Recharge started.", "Recharge finished."]
POWER_ADAPTERS: dict[str, str] = {"Android": "MicroUSB", "iPhone": "Lightning"}
CONNECTED_MSG: str = "{} connected."
CONNECT_FIRST_MSG: str = "Connect {} first."

class RechargeTemplate(metaclass = ABCMeta):
    @abstractmethod
    def recharge(self) -> NoReturn:
        raise NotImplementedError("You should implement this.")

class FormatIPhone(RechargeTemplate):
    @abstractmethod
    def use_lightning(self) -> NoReturn:
        raise NotImplementedError("You should implement this.")

class FormatAndroid(RechargeTemplate):
    @abstractmethod
    def use_micro_usb(self) -> NoReturn:
        raise NotImplementedError("You should implement this.")

class IPhone(FormatIPhone):
    __name__: str = "iPhone"

    def __init__(self):
        self.connector: bool = False

    def use_lightning(self) -> None:
        self.connector = True
        print(CONNECTED_MSG.format(POWER_ADAPTERS[self.__name__]))

    def recharge(self) -> None:
        if self.connector:
            for state in RECHARGE:
                print(state)
        else:
            print(CONNECT_FIRST_MSG.format(POWER_ADAPTERS[self.__name__]))

class Android(FormatAndroid):
    __name__: str = "Android"

    def __init__(self) -> None:
        self.connector: bool = False

    def use_micro_usb(self) -> None:
        self.connector = True
        print(CONNECTED_MSG.format(POWER_ADAPTERS[self.__name__]))

    def recharge(self) -> None:
        if self.connector:
            for state in RECHARGE:
                print(state)
        else:
            print(CONNECT_FIRST_MSG.format(POWER_ADAPTERS[self.__name__]))

class IPhoneAdapter(FormatAndroid):
    def __init__(self, mobile: FormatAndroid) -> None:
        self.mobile: FormatAndroid = mobile

    def recharge(self) -> None:
        self.mobile.recharge()

    def use_micro_usb(self) -> None:
        print(CONNECTED_MSG.format(POWER_ADAPTERS["Android"]))
        self.mobile.use_lightning()

class AndroidRecharger:
    def __init__(self) -> None:
        self.phone: Android = Android()
        self.phone.use_micro_usb()
        self.phone.recharge()

class IPhoneMicroUSBRecharger:
    def __init__(self) -> None:
        self.phone: IPhone = IPhone()
        self.phone_adapter: IPhoneAdapter = IPhoneAdapter(self.phone)
        self.phone_adapter.use_micro_usb()
        self.phone_adapter.recharge()

class IPhoneRecharger:
    def __init__(self) -> None:
        self.phone: IPhone = IPhone()
        self.phone.use_lightning()
        self.phone.recharge()

print("Recharging Android with MicroUSB recharger.")
AndroidRecharger()
print()

print("Recharging iPhone with MicroUSB using adapter pattern.")
IPhoneMicroUSBRecharger()
print()

print("Recharging iPhone with iPhone recharger.")
IPhoneRecharger()
</syntaxhighlight>

==== C# ====
<syntaxhighlight lang="c#">
namespace Wikipedia.Examples;

using System;

interface ILightningPhone
{
	void ConnectLightning();
	void Recharge();
}

interface IUsbPhone
{
	void ConnectUsb();
	void Recharge();
}

sealed class AndroidPhone : IUsbPhone
{
	private bool isConnected;

	public void ConnectUsb()
	{
		this.isConnected = true;
		Console.WriteLine("Android phone connected.");
	}

	public void Recharge()
	{
		if (this.isConnected)
		{
			Console.WriteLine("Android phone recharging.");
		}
		else
		{
			Console.WriteLine("Connect the USB cable first.");
		}
	}
}

sealed class ApplePhone : ILightningPhone
{
	private bool isConnected;

	public void ConnectLightning()
	{
		this.isConnected = true;
		Console.WriteLine("Apple phone connected.");
	}

	public void Recharge()
	{
		if (this.isConnected)
		{
			Console.WriteLine("Apple phone recharging.");
		}
		else
		{
			Console.WriteLine("Connect the Lightning cable first.");
		}
	}
}

sealed class LightningToUsbAdapter : IUsbPhone
{
	private readonly ILightningPhone lightningPhone;

	private bool isConnected;

	public LightningToUsbAdapter(ILightningPhone lightningPhone)
	{
		this.lightningPhone = lightningPhone;
	}

	public void ConnectUsb()
	{
		this.lightningPhone.ConnectLightning();
	}

	public void Recharge()
	{
		this.lightningPhone.Recharge();
	}
}

public class AdapterDemo
{
    static void Main(string[] args)
    {
	    ILightningPhone applePhone = new ApplePhone();
	    IUsbPhone adapterCable = new LightningToUsbAdapter(applePhone);
	    adapterCable.ConnectUsb();
	    adapterCable.Recharge();
    }
}
</syntaxhighlight>Output:<syntaxhighlight lang="output">
Apple phone connected.
Apple phone recharging.
</syntaxhighlight>

==See also==
* [https://java-design-patterns.com/patterns/adapter/ Adapter] Java Design Patterns - Adapter
* [[Delegation (programming)|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 (computing)|Shim]]
* [[Wrapper function]]
* [[Wrapper library]]

==References==
{{Reflist|refs=
<ref name="GoF">{{cite book |author-first1=Erich |author-last1=Gamma |author-first2=Richard |author-last2=Helm |author-first3=Ralph |author-last3=Johnson |author-first4=John |author-last4=Vlissides |title=Design Patterns: Elements of Reusable Object-Oriented Software |date=1994 |publisher=[[Addison Wesley]] |isbn=0-201-63361-2 |pages=[https://archive.org/details/designpatternsel00gamm/page/139 139ff] |url-access=registration |url=https://archive.org/details/designpatternsel00gamm/page/139 }}</ref>

<ref name="Adapter_Design_Pattern">{{cite web |title=The Adapter design pattern - Problem, Solution, and Applicability |url=http://w3sdesign.com/?gr=s01&ugr=proble |website=w3sDesign.com |access-date=2017-08-12 |archive-url=https://web.archive.org/web/20170828230927/http://w3sdesign.com/?gr=s01&ugr=proble |archive-date=2017-08-28}}</ref>

<ref name="Structure">{{cite web |title=The Adapter design pattern - Structure and Collaboration |url=http://w3sdesign.com/?gr=s01&ugr=struct |website=w3sDesign.com |access-date=2017-08-12 |archive-url=https://web.archive.org/web/20170828231911/http://w3sdesign.com/?gr=s01&ugr=struct |archive-date=2017-08-28}}</ref>

<ref name="HeadFirst">{{cite book |author-last1=Freeman |author-first1=Eric |author-last2=Freeman |author-first2=Elisabeth |author-first3=Kathy |author-last3=Sierra |author-link3=Kathy Sierra |author-last4=Bates |author-first4=Bert |date=2004 |title=Head First Design Patterns |pages=244 |publisher=[[O'Reilly Media]] |format=paperback |isbn=978-0-596-00712-6 |access-date=2013-04-30 |oclc=809772256 |url=http://www.headfirstlabs.com/books/hfdp/ |archive-url=https://web.archive.org/web/20130504183857/http://www.headfirstlabs.com/books/hfdp/ |archive-date=2013-05-04 |url-status=dead }}</ref>
}}

{{Commons category}}
{{Wikibooks|Computer Science Design Patterns|Adapter|Adapter implementations in various languages}}

{{Design Patterns Patterns}}

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