package exponenta;

import java.util.function.BinaryOperator;

public class Iter {
	
	@FunctionalInterface
	interface Fi<F, A, B, I> {
		
		I iterate(F f, A a ,B b, I i);
	}
	
    static Fi<BinaryOperator<Integer>, Integer, Integer, Integer> fi;
    static BinaryOperator<Integer> fact = (x, y) -> { return x * y; };

    public static void main(String[] args) {

    	fi = (BinaryOperator<Integer> f, Integer a, Integer b, Integer i) -> {
        	if(a > b) return i;
        	else return f.apply(fi.iterate(f, a + 1, b, i), a);
        };
        
        System.out.println(fi.iterate(fact, 1, 5, 1));
    }
    
}

package iter;

import static java.lang.System.out;

public class Iter {

    public static void main(String[] args) {
        out.println(fi(1, 5));
    }

    static int fi(int a, int b) {
        return a > b ? 1 : fi(a + 1, b) * a;
    }
}