Understanding the bridge design pattern

design-patterns

I do not understand the "bridge" design pattern at all. I have gone through various web sites, but they haven't helped.

Can anybody help me in understanding this?

Best Answer

In OOP we use polymorphism so an abstraction can have multiple implementations. Let's look at the following example:

//trains abstraction
public interface Train
{ 
    move();
}
public class MonoRail:Train
{
    public override move()
    {
        //use one track;
    }
}
public class Rail:Train
{
    public override move()
    {
        //use two tracks;
    }
}

A new requirement introduced and needs to bring in the acceleration perspective of the trains, so change the code as below.

    public interface Train
    { 
        void move();
    }
    public class MonoRail:Train
    {
        public override void move()
        {
            //use one track;
        }
    }
    public class ElectricMonoRail:MonoRail
    {
        public override void move()
        {
            //use electric engine on one track.
        }
    }
    public class DieselMonoRail: MonoRail
    {
        public override void move()
        {
            //use diesel engine on one track.
        }
    }
    public class Rail:Train
    {
        public override void move()
        {
            //use two tracks;
        }
    }
    public class ElectricRail:Rail
    {
        public override void move()
        {
            //use electric engine on two tracks.
        }
    }
    public class DieselRail: Rail
    {
        public override void move()
        {
            //use diesel engine on two tracks.
        }
    }

The above code is not maintainable and lacks reusability (assuming we could reuse the acceleration mechanism for the same track platform). The following code applies the bridge pattern and separates the two different abstractions, train transport and acceleration. 

public interface Train
{ 
    void move(Accelerable engine);
}
public interface Accelerable
{
    public void accelerate();
}
public class MonoRail:Train
{
    public override void move(Accelerable engine)
    {
        //use one track;
        engine.accelerate(); //engine is pluggable (runtime dynamic)
    }
}
public class Rail:Train
{
    public override void move(Accelerable engine)
    {
        //use two tracks;
        engine.accelerate(); //engine is pluggable (runtime dynamic)
    }
}
public class ElectricEngine:Accelerable{/*implementation code for accelerable*/}
public class DieselEngine:Accelerable{/*implementation code for accelerable*/}
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