Explain the lambda expression

Question

I have a method that contains a lambda expression:

public int noOfComplementaryPairs1(int arr[], int k) {

    Map<Integer, Integer> map = new HashMap<Integer, Integer>();

    for (int i = 0; i < arr.length; i++) {
        map.merge(k - arr[i], 1, Integer::sum);
    }

    return Arrays.stream(arr).map(element -> map.getOrDefault(element, 0)).sum();
}

Can someone explain to me what the last 2 lines mean? I am not new to Java, but sometimes I get confused with some expressions like this one.

Eugene · Accepted Answer · 2017-11-06 04:58:59Z

3

map.merge... : the first param is the key in the map, the second param is the value in the map when this key is not present, the third one is a BiFunction that tells u how to merge two keys, since u cant have two keys with the same value inside a Map..

The last line is pretty simple too: you are streaming the array, mapping each element of that array with: map.getOrDefault(element, 0), which means get the value from the map with key = element or the default of zero if it is not present; then sum them all.

edited Nov 6, 2017 at 4:58

answered Nov 6, 2017 at 4:55

Eugene

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3 Comments

Arefe Over a year ago

How about the lambda expression? It will be helpful to upvote the question btw

Naman Over a year ago

How about using

Map<Integer, Integer> map =                 IntStream.range(0, arr.length).boxed().collect(Collectors.toMap(i -> k - arr[i], i -> 1, Integer::sum));

instead of the for loop? Any pros or cons you might want to add to the answer.

Eugene Over a year ago

@nullpointer hardly do I agree here, way less readable; but ultimately it's the OP to decide

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