Concatenate two list by applying an operation in pairs using java stream

Question

I have two lists of Boolean and I need to concatenate them by applying an AND operation to the elements with the same index, I'm expecting to get a list of Booleans that contains the result of doing the operation in pairs.

public List<Boolean> concatenate(List<Boolean> l1, List<Boolean> l2) {

    return l1.stream()
                .flatMap(e1 -> l2.stream()
                                .filter(e2-> l1.indexOf(e1) == l2.indexOf(e2))
                                .map(e2-> e1&&e2))
                .collect(Collectors.toList());
}

The size of the resulting list will be l1.size()*l2.size() so the filter inside the second stream is filtering anything.

Do you must use streams? It is just making your life complicated. Why don't you use classical loop? — tdavid, Apr 18 '20 at 20:22
l1 : {true,false,true}, l2 : {false,true,true} -> {false,false,true} — TonyRomero, Apr 18 '20 at 20:26
what if the two list have different number of elements? what do you expect then? — tdavid, Apr 18 '20 at 20:28

score 3 · Answer 1 · answered Apr 18 '20 at 20:54

You can easily do it using com.google.guava library (version 21 and above)

public List<Boolean> concatenate(List<Boolean> firstList, List<Boolean> secondList) {
        return Streams.zip(firstList.stream(), secondList.stream(), (a, b) -> (a && b))
                .collect(Collectors.toList());
    }

score 1 · Accepted Answer · answered Apr 18 '20 at 20:35

List<Boolean> l1 = new ArrayList<>(Arrays.asList(true, false, true));
List<Boolean> l2 = new ArrayList<>(Arrays.asList(false, true, true));

List<Boolean> l3 = IntStream.range(0, min(l1.size(), l2.size()))
                            .mapToObj(i -> l1.get(i)&&l2.get(i))
                            .collect(Collectors.toList());

// [false, false, true]

You can use IntStream.range() to iterate over both the lists at once and .mapToObj() to convert the && to Boolean and .collect() to store it in a new List.

You can simplify the range to .range(0, l1.size()) if you know for sure that both of them are of the same size.

score 1 · Answer 3 · answered Apr 18 '20 at 20:37

The problem with the filtering is that you use indexOf which always return the first occurrence of the element, since you have Booleans.

Given that the size of the two lists are equal, you can go with the classical way:

public List<Boolean> concatenate(List<Boolean> l1, List<Boolean> l2) {
    List<Boolean> result = new ArrayList<>();
    for(int index = 0; index < l1.size(); index ++) {
        result.add(l1.get(index) && l2.get(index));
    }
    return result; 
}

The OP has asked for a solution with streams. This is probably not what the OP is looking for. — Harshal Parekh, Apr 18 '20 at 20:38

Concatenate two list by applying an operation in pairs using java stream

3 Answers3