Does the Bridge Pattern decouples an abstraction from implementation?

Question

I learned Bridge pattern from different articles and I have implemented that as per my understanding . One thing that is confusing me is bridge pattern says

BridgePattern decouples an abstraction from its implementation so that the two can vary independently

what is meaning of this statement? Is implementation resides at in separate jar ?

what is meaning of vary independently statement ?

considering the provided journaldev article, elaborate the answer.

Any help is greatly appreciated.

Answer 1

BridgePattern decouples an abstraction from its implementation.

Abstraction and Implementation can vary independently since the concrete class does not directly implement Abstraction ( interface)

Key note: Two orthogonal class hierarchies (The Abstraction hierarchy and Implementation hierarchy) are linked using composition (and not inheritance).This composition helps both hierarchies to vary independently.

Implementation never refers Abstraction. Abstraction contains Implementation interface as a member (through composition).

Coming back to your question regarding the example code in journaldev article :

Shape is Abstraction

Triangle is RedefinedAbstraction

Color is Implementor

RedColor is ConcreteImplementor

A concrete Shape object : Triangle extends Shape but does not implement the Color interface.

public class Triangle extends Shape{
}

RedColor and GreenColor actually implement the Color interface.

The Concrete Shape object (Triangle) is independent of implementing abstraction (i.e. Color interface).

Shape tri = new Triangle(new RedColor());

Here Triangle contains a concrete Color object ( Composition). If there is a change in the Color abstraction (interface), RedColor and GreenColor are responsible for implementing the abstraction of Color interface.

Shapes like Triangle is not affected by changes in contract to the Color interface. So the Color interface can vary independently. This is possible because Shape holds the contract that uses Composition rather than implementation.

In Summary,

1. Bridge is a structural pattern
2. Abstraction and implementation are not bound at compile time
3. Abstraction and implementation - both can vary without impact in client

Use the Bridge pattern when:

1. You want run-time binding of the implementation,
2. You have a proliferation of classes from a coupled interface and numerous implementations,
3. You want to share an implementation among multiple objects,
4. You need to map orthogonal class hierarchies.

Useful links:

tutorialspoint artice

dzone article

oodesign article

sourcemaking article

Related post:

When do you use the Bridge Pattern? How is it different from Adapter pattern?

Answer 2

This statement simply means, that you can switch implementor, to which abstraction points to, in run-time and everything should work (like in Strategy Pattern; but in Strategy Pattern, only strategies are abstract). It can be also understood as separating two classes, so that they don't have to know about each other more than just their interfaces.