java design pattern - bridge pattern

According to GoF definition:　bridge pattern is ,decouple an abstraction from its implementation 
so that the two can vary independently. 
我实在是理解无力， 还是看code吧。

Usage in Jdk:
AWT (It provides an abstraction layer which maps onto the native OS the windowing support)
JDBC

package BridgePattern;

interface VehicleType{
    abstract public void book();
}

abstract class Vehicle{
    protected VehicleType type1;
    protected VehicleType type2;

    public Vehicle(VehicleType type1, VehicleType type2){
        this.type1 = type1;
        this.type2 = type2;
    }

    abstract public void purchase();
}


class Car extends Vehicle{
    //Constructor
    public Car(VehicleType type1, VehicleType type2){
        super(type1, type2);
    }

    @Override
    public void purchase(){
        System.out.println("car");
        type1.book();
        type2.book();
    }
}

class Bike extends Vehicle{
    public Bike(VehicleType type1, VehicleType type2){
        super(type1, type2);
    }

    @Override
    public void purchase(){
        System.out.println("Bike");
        type1.book();
        type2.book();
    }
}


class NewVehicle implements VehicleType{

    @Override
    public void book(){
        System.out.println("New Vehicle");
    }
}

class OldVehicle implements VehicleType{
    @Override
    public void book(){
        System.out.println("Old Vehicle");
    }
}

public class BridgePatternDemo {
    public static void main(String[] args){
        Vehicle vehicle1 = new Car(new NewVehicle(), new OldVehicle());
        vehicle1.purchase();

        Bike bike1 = new Bike(new NewVehicle(), new OldVehicle());
        bike1.purchase();
    }
}