Generate all possible keys for 2 or more sets of strings in same order

There are N sets with M values in each. How to generate all possible combination in given order of sets efficiently?.

SetA = {'A', 'B'}
SetB = {'x', 'Y'}
SetC = {'L', 'M'}

Key Order = SetA_SetB_SetC

Output :

A_X_L, A_X_M, A_Y_L, A_Y_M, B_X_L, B_X_M, B_Y_L, B_Y_M

From Java-8 onwards use #Stream API, I have implemented the code below:

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;


public class CandidateCode {

    public static void main(String[] args) {
        List<String> ListA = Arrays.asList("A", "B");
        List<String> ListX = Arrays.asList("X", "Y");
        List<String> ListL = Arrays.asList("L", "M");
        List<String> collect = ListA.stream().flatMap(a -> ListX.stream().flatMap(x -> ListL.stream().map(l -> a + "_" + x + "_" + l))).collect(Collectors.toList());
        collect.forEach(System.out::println);
    }
}

Output:

A_X_L
A_X_M
A_Y_L
A_Y_M
B_X_L
B_X_M
B_Y_L
B_Y_M