How to provide parameter to Google's Guice Provider?

Question

Lets assume the UI is supposed to pass either 2 or 4 depending on what choice user makes. The backend is using a Provider from Guice to instantiate the valid class. However the 'get' itself does not take in any parameter, however I want to do something like the sameple code below. How can this be accomplished ?

public class VehicleProvider implements Provider<Vehicle> {

    public Vehicles get(int numberOfTyres) {
        /**
         * numberTyres == 2
         *    return new TwoWheelVehicles
         * numberTyres == 4
         *    return new FourWheelVehicles
         */
    }
}

Answer 1

To do the thing most similar to "passing a parameter to a provider", you could define a factory class, for example:

class VehicleFactory {
  Vehicle build(int numWheels) {
    return new Vehicle(numWheels);
  }
}

and then inject this factory to the place which needs to create the instances of Vehicle.

You could consider using Assisted injection to define the bindings for the factory; assisted injection confuses me a lot whenever I use it, so you may want to consider simpler solutions first.

If you actually only need to create 2- or 4-wheeled vehicles (and not some other arbitrary number), you could give your factory specific methods:

class VehicleFactory {
  Vehicle buildTwoWheeled() {
    return new Vehicle(2);
  }

  Vehicle buildFourWheeled() {
    return new Vehicle(4);
  }
}

Or you could bind separate instances of the provider:

public class TwoWheelVehicleProvider implements Provider<Vehicle> { ... }
public class FourWheelVehicleProvider implements Provider<Vehicle> { ... }

Or, define binding annotations:

@Provides @TwoWheeled
Vehicle provideTwoWheeledVehicle() { return new Vehicle(2); }

@Provides @FourWheeled
Vehicle provideFourWheeledVehicle() { return new Vehicle(4); }