Bridge Pattern Explained with Code and Real-World Scenarios

Bridge Pattern

Definition and purpose

Bridge separates two independent varying dimensions of a concept, connecting them through a bridge rather than creating a subclass for every combination. The goal is to reduce class count and allow each side to evolve independently.

Typical scenarios

Pen size (big/medium/small) combined with color (red/black/blue) – avoid classes like BigRedPen.

Software: multiple platforms (Android/iOS) with multiple themes (light/dark); message type (SMS/WeChat) with delivery channel.

Video player: format (MP4/AVI) with operating system (Windows/Mac).

Java implementation (Pen + Color)

1. Color dimension (independent variation layer)

// Color interface
public interface Color {
    void showColor();
}

// Red
public class Red implements Color {
    @Override
    public void showColor() {
        System.out.print("红色");
    }
}

// Black
public class Black implements Color {
    @Override
    public void showColor() {
        System.out.print("黑色");
    }
}

2. Pen abstraction (holds a Color as bridge)

public abstract class Pen {
    protected Color color;
    public Pen(Color color){
        this.color = color;
    }
    public abstract void draw();
}

3. Concrete pen variants (independent extensions)

// Big pen
public class BigPen extends Pen{
    public BigPen(Color color) {
        super(color);
    }
    @Override
    public void draw() {
        color.showColor();
        System.out.println("大号画笔");
    }
}

// Small pen
public class SmallPen extends Pen{
    public SmallPen(Color color) {
        super(color);
    }
    @Override
    public void draw() {
        color.showColor();
        System.out.println("小号画笔");
    }
}

4. Test invocation

public class Test {
    public static void main(String[] args) {
        Pen bigRed = new BigPen(new Red());
        bigRed.draw();
        Pen smallBlack = new SmallPen(new Black());
        smallBlack.draw();
    }
}

Common business use cases

Objects with two or more independent varying dimensions that need continuous extension.

When multi‑layer combinations would generate many subclasses, use Bridge to avoid class explosion.

Multi‑platform clients: separate system layer from business functionality layer.

Multimedia tools: decouple file format from device system.

Message push: separate message type from delivery channel.

Distinguishing from Adapter

Adapter addresses post‑hoc interface compatibility, while Bridge separates dimensions proactively during design.