Before we define the NeuralNetwork class, let's take a look at an example of Java code with interface:

public interface IActivationFunction {
  double calc(double x);
  public enum ActivationFunctionENUM {
    STEP, LINEAR, SIGMOID, HYPERTAN
  }
}

The calc() signature method is used by a specific Activation Function that implements this interface, the Sigmoid function, for example:

public class Sigmoid implements IActivationFunction {
    private double a=1.0;
public Sigmoid(double _a){this.a=_a;}
@Override
    public double calc(double x){
        return 1.0/(1.0+Math.exp(-a*x));
    }
}

This is one example of polymorphism, whereby a class or method may present different behavior, but yet under the same signature, allowing a flexible application.