Euler's Method Minimal

public class EulersMethod {
    public static double eulersMethod(FPrime<Double,Double> func, double initX, double initY, double stepSize, double target) {
        if(target<initX+0.000000000000001) return initY;
        double lastVal = eulersMethod(func, initX, initY, stepSize, target-stepSize);
        return lastVal + stepSize*func.val(target-stepSize, lastVal);
    }
}

public interface FPrime<X,Y> {
    double val(X x,Y y);
}