Question

I’m writing a program to simulate an artificial neural network. I have the following classes and interfaces set up:

public interface Neuron
{
}

// Input neuron
public class INeuron implements Neuron
{
}

// Output and hidden neuron
public class ONeuron implements Neuron
{
}

public interface Layer
{
    public ArrayList<Neuron> getNeurons();
}

// Input layer
public class ILayer implements Layer
{
    ArrayList<INeuron> neurons = new ArrayList<INeuron>();

    public ArrayList<Neuron> getNeurons()
    {
        return neurons;
    }

    // other stuff appropriate to the input layer
}

The compiler reports “cannot convert from ArrayList<INeuron> to ArrayList<Neuron>.”

I’ve tried switching things around. For example: ArrayList<Neuron> neurons = new ArrayList<INeuron>(). But that just seems to shift the same error to different parts of the class.

I don’t understand why INeuron can’t be implicitly cast to Neuron since INeuron is a subtype of Neuron.

Answer 1

You need to use:

public interface Layer
{
    public ArrayList<? extends Neuron> getNeurons();
}

because of lack of covariance and contravariance in generics. If you return an ArrayList < INeuron > as an ArrayList < Neuron > then you would potentially be able to add an element that is not a INeuron (like another child of Neuron) to an ArrayList < INeuron >