Definite Integral in Java

Function.java

public interface Function {

    public double f(double x);

}

MainWithLambda.java

public class MainWithLambda {

    public static final double INCREMENT = 1E-4;

    public static void main(String[] args) {
        System.out.println(integral(0, 2, x -> {
            return Math.pow(x, 2);
        }));
    }

    public static double integral(double a, double b, Function function) {
        double area = 0;
        double modifier = 1;
        if(a > b) {
            double tempA = a;
            a = b;
            b = tempA;
            modifier = -1;
        }
        for(double i = a + INCREMENT; i < b; i += INCREMENT) {
            double dFromA = i - a;
            area += (INCREMENT / 2) * (function.f(a + dFromA) + function.f(a + dFromA - INCREMENT));
        }
        return (Math.round(area * 1000.0) / 1000.0) * modifier;
    }

}

MainWithoutLambda.java

public class MainWithoutLambda {

    public static final double INCREMENT = 1E-4;

    public static void main(String[] args) {
        System.out.println(integral(0, 2, new Function() {
            @Override
            public double f(double x) {
                return Math.pow(x, 2);
            }
        }));
    }

    public static double integral(double a, double b, Function function) {
        double area = 0;
        double modifier = 1;
        if(a > b) {
            double tempA = a;
            a = b;
            b = tempA;
            modifier = -1;
        }
        for(double i = a + INCREMENT; i < b; i += INCREMENT) {
            double dFromA = i - a;
            area += (INCREMENT / 2) * (function.f(a + dFromA) + function.f(a + dFromA - INCREMENT));
        }
        return (Math.round(area * 1000.0) / 1000.0) * modifier;
    }

}