Wrapper

Convert the interface of a class into another interface the clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.

Real world example

Consider that you have some pictures in your memory card and you need to transfer them to your computer. In order to transfer them you need some kind of adapter that is compatible with your computer ports so that you can attach memory card to your computer. In this case card reader is an adapter. Another example would be the famous power adapter; a three legged plug can't be connected to a two pronged outlet, it needs to use a power adapter that makes it compatible with the two pronged outlet. Yet another example would be a translator translating words spoken by one person to another

In plain words

Adapter pattern lets you wrap an otherwise incompatible object in an adapter to make it compatible with another class.

Wikipedia says

In software engineering, the adapter pattern is a software design 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.

Programmatic Example

Consider a captain that can only use rowing boats and cannot sail at all.

First we have interfaces RowingBoat and FishingBoat

public interface RowingBoat {
  void row();
}

public class FishingBoat {
  private static final Logger LOGGER = LoggerFactory.getLogger(FishingBoat.class);
  public void sail() {
    LOGGER.info("The fishing boat is sailing");
  }
}

And captain expects an implementation of RowingBoat interface to be able to move

public class Captain implements RowingBoat {

  private RowingBoat rowingBoat;

  public Captain(RowingBoat rowingBoat) {
    this.rowingBoat = rowingBoat;
  }

  @Override
  public void row() {
    rowingBoat.row();
  }
}

Now let's say the pirates are coming and our captain needs to escape but there is only fishing boat available. We need to create an adapter that allows the captain to operate the fishing boat with his rowing boat skills.

public class FishingBoatAdapter implements RowingBoat {

  private static final Logger LOGGER = LoggerFactory.getLogger(FishingBoatAdapter.class);

  private FishingBoat boat;

  public FishingBoatAdapter() {
    boat = new FishingBoat();
  }

  @Override
  public void row() {
    boat.sail();
  }
}

And now the Captain can use the FishingBoat to escape the pirates.

Captain captain = new Captain(new FishingBoatAdapter());
captain.row();

Use the Adapter pattern when

• you want to use an existing class, and its interface does not match the one you need
• you want to create a reusable class that cooperates with unrelated or unforeseen classes, that is, classes that don't necessarily have compatible interfaces
• you need to use several existing subclasses, but it's impractical to adapt their interface by subclassing every one. An object adapter can adapt the interface of its parent class.
• most of the applications using third party libraries use adapters as a middle layer between the application and the 3rd party library to decouple the application from the library. If another library has to be used only an adapter for the new library is required without having to change the application code.

Class and object adapters have different trade-offs. A class adapter

• adapts Adaptee to Target by committing to a concrete Adaptee class. As a consequence, a class adapter won’t work when we want to adapt a class and all its subclasses.
• let’s Adapter override some of Adaptee’s behavior, since Adapter is a subclass of Adaptee.
• introduces only one object, and no additional pointer indirection is needed to get to the adaptee.

An object adapter

• let’s a single Adapter work with many Adaptees—that is, the Adaptee itself and all of its subclasses (if any). The Adapter can also add functionality to all Adaptees at once.
• makes it harder to override Adaptee behavior. It will require subclassing Adaptee and making Adapter refer to the subclass rather than the Adaptee itself.

• java.util.Arrays#asList()
• java.util.Collections#list()
• java.util.Collections#enumeration()
• javax.xml.bind.annotation.adapters.XMLAdapter

• Design Patterns: Elements of Reusable Object-Oriented Software
• J2EE Design Patterns