Java 8 Lambda - Intersection of Two Lists

Question

I am trying to find intersection of two lists based on some condition and doing some steps. Couldn't find a way to do it (in learning stage) :)

Double totalAmount = 0.00d;
Double discount = 0.00d;


List<OrderLineEntry> orderLineEntryList = orderEntry.getOrderReleases().stream()
    .flatMap(orderReleaseEntry -> orderReleaseEntry.getOrderLines().stream())
    .filter(orderLineEntry -> orderLineEntry.getStatus().equals("PP") || orderLineEntry.getStatus().equals("PD"))
    .collect(Collectors.toList());

for (OrderLineEntry orderLineEntry : orderLineEntryList) {
    for (SplitLineEntry splitLineEntry : splitReleaseEntry.getLineEntries()) {
        if (splitLineEntry.getOrderLineId().equals(orderLineEntry.getId()) && splitLineEntry.getStatusCode() != "PX") {
            totalAmount += orderLineEntry.getFinalAmount();
            couponDiscount += orderLineEntry.getCouponDiscount() == null ? 0.00d : orderLineEntry.getCouponDiscount();
        }
    }
}

As you see, the logic is simple

Get All items from order based on some filter list and intersect with another list and do some stuff.

The most efficient way to find an intersection is to use a Set or a Map. I suggest you build a set by collecting an appropriate groupingBy. — Peter Lawrey, Jul 28 '15 at 17:41
@PeterLawrey, Can you help me achieve the same with Lambda. I am creating list just to find intersection. Typical Java coding, I would use `Map` :) — RaceBase, Jul 29 '15 at 04:23

score 143 · Accepted Answer · edited Jul 28 '15 at 17:49

143

The simplest approach is this:

List<T> intersect = list1.stream()
                         .filter(list2::contains)
                         .collect(Collectors.toList());

edited Jul 28 '15 at 17:49

Makoto

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answered Jul 28 '15 at 17:46

Silas Reinagel

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I have seen this example. How do i use this for my use case. Since `list1` and list2 are of different types and I need to compare them on assume `list1.id == list2.fk_id` – RaceBase Jul 29 '15 at 04:24
8

As an optimisation I would turn list2 into a HashSet first. – Peter Lawrey Jul 29 '15 at 04:59
6

contains is O(n), so this is a O(n^2) set intersection operation – fairidox Aug 15 '16 at 21:39
1

I have tried this, but I think ::contains use == instead of equals, the result intersection may be empty. – Maxi Wu Dec 21 '16 at 09:06
3

We need to implement HashCode & equals for T, in order to intersect works fine. – ben rhouma moez Nov 07 '17 at 09:46
I would rather use Set as the outcome to eliminate duplicates. You could also add `.filter(Objects::nonNull)` to avoid null values if you want. – Saša Oct 18 '19 at 10:20

score 14 · Answer 2 · answered Jul 29 '15 at 05:10

I need to compare them on assume list1.id == list2.fk_id

First build up a set of the fk_id;

Set<Integer> orderLineEntrSet = orderEntry.getOrderReleases().stream()
    .flatMap(orderReleaseEntry ->
orderReleaseEntry.getOrderLines().stream())
    .filter(orderLineEntry -> { 
            String s = orderLineEntry.getStatus(); 
            return "PP".equals(s) || "PD".equals(s); 
    })
    .map(e -> e.getId())
    .collect(Collectors.toSet());

double[] totalAmount = { 0.0 };
double[] couponDiscount = { 0.0 };
orderLineEntryList.stream()
    .flatMap(sre -> sre.getLineEntries().stream())
    .filter(ole -> orderLineEntrySet.contains(ole.getOrderLineId())
    .filter(ole -> !"PX".equals(ole.getStatusCode()))
    .forEach(ole -> {
            totalAmount[0] += ole.getFinalAmount();
            if (ole.getCouponDiscount() != null)
                couponDiscount[0] += ole.getCouponDiscount();
        });

You can avoid using a reference to an array object by using reduce function. e.g. see how Collectors.averagingDouble is implemented. But I find this more complicated.

Note: this is O(N) by using a set of ids rather than using a list of matching ids which would be O(N^2)

Not sure about the O(N). Since you need to do a 'contains' on the second list, I'd think this is still O(N^2). — HakunaM, Jan 05 '17 at 18:27
@HakunaM Set.contains using a hash set is O(1) amortized (and O(log N) for a tree set) Collectors.toSet() returns a hash set. — Peter Lawrey, Jan 06 '17 at 14:58

score -8 · Answer 3 · edited Jan 14 '19 at 20:33

-8

List<T> intersect = list1.stream().filter(set1::contains).collect(Collectors.toList());

This will work as long as T is a String, Integer, Float, etc. where the equals and HashCode methods are straightforward. But if the T is a custom object, we need to implement HashCode & equals

edited Jan 14 '19 at 20:33

Erik

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answered Jul 26 '16 at 23:42

psekar

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this answer, added a year later, is no different than top voted answer other than being more wordy and less formatted.. – harschware Nov 16 '17 at 04:37

Java 8 Lambda - Intersection of Two Lists

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