ProductOfNumbers.java

import java.util.ArrayList;
import java.util.List;

public class ProductOfNumbers {

    private List<Integer> list;

    public ProductOfNumbers() {
        list = new ArrayList<>();
    }

    public void add(int num) {
        list.add(num);
    }

    public int getProduct(int k) {
        int num=1, size= list.size();
        for(int i=size-1; i>=size-k;i--) {
            num = num* list.get(i);
        }
        return num;
    }

    public static void main(String[] args) {
        ProductOfNumbers productOfNumbers = new ProductOfNumbers();
        productOfNumbers.add(3);        // [3]
        productOfNumbers.add(0);        // [3,0]
        productOfNumbers.add(2);        // [3,0,2]
        productOfNumbers.add(5);        // [3,0,2,5]
        productOfNumbers.add(4);        // [3,0,2,5,4]
        System.out.println(productOfNumbers.getProduct(2));
        System.out.println(productOfNumbers.getProduct(3));
        System.out.println(productOfNumbers.getProduct(4));
        productOfNumbers.add(8);
        System.out.println(productOfNumbers.getProduct(2));
    }
}