Difference between Bridge pattern and adapter pattern.

Question

What is difference between Bridge and Adapter pattern where i can use which pattern.

Answer 1

http://en.wikipedia.org/wiki/Adapter%5Fpattern

The Adapter pattern is more about getting your existing code to work with a newer system or interface.

If you have a set of company-standard web service APIs that you'd like to offer to another application's existing extensibility interface, you might consider writing a set of adapters to do this. Note that there's a grey area and this is more about how you technically define the pattern, since other patterns like the facade are similar.

http://en.wikipedia.org/wiki/Bridge%5Fpattern

The Bridge pattern is going to allow you to possibly have alternative implementations of an algorithm or system.

Though not a classic Bridge pattern example, imagine if you had a few implementations of a data store: one is efficient in space, the other is efficient in raw performance... and you have a business case for offering both in your app or framework.

In terms of your question, "where I can use which pattern," the answer is, wherever it makes sense for your project! Perhaps consider offering a clarification edit to guide the discussion on where you believe you need to use one or the other.

Answer 2

"Adapter makes things work after they're designed; Bridge makes them work before they are. [GoF, p219]"

Effectively, the Adapter pattern is useful when you have existing code, be it third party, or in-house, but out of your control, or otherwise not changeable to quite meet the interface you need it to. For instance, we have a SuperWeaponsArray which can control a fine array of doomsday devices.

public class SuperWeaponsArray {
  /*...*/

  public void destroyWorld() {
    for (Weapon w : armedWeapons) {
      w.fire();
    }
  }
}

Great. Except we realize we have a nuclear device in our arsenal that vastly predates the conversion to the Weapon interface. But we'd really like it to work here... so what do we do... wedge it in!

NukeWeaponsAdaptor - based off of our Nuke class, but exporting the Weapon interface. Sweet, now we can surely destroy the world. It seems like bit of a kludge, but it makes things work.

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The Bridge pattern is something you implement up front - if you know you have two orthogonal hierarchies, it provides a way to decouple the interface and the implementation in such a way that you don't get an insane number of classes. Let's say you have:

MemoryMappedFile and DirectReadFile types of file objects. Let's say you want to be able to read files from various sources (Maybe Linux vs. Windows implementations, etc.). Bridge helps you avoid winding up with:

MemoryMappedWindowsFile MemoryMappedLinuxFile DirectReadWindowsFile DirectReadLinuxFile