Find intersection between collections with different objects

Question

I have list of custom objects like below:

List<CustomObject> existingValues

And another collection with ids that has been sent from client and contains ids of CustomObject mentoioned above Set<Long> ids.

My goal is to return collection with CustomObject that will contains only elements where ids intersect.

I can simply do this with nested for each cycles. But it looks a bit ugly.

In SQL I can do similar stuff with query:

select * where customobject.id in (some ids...)

In wich way it can be achived with lamdaj or guava?

This isn't an answer since you're asking about lamdaj and guava, but really you only need a single loop through the CustomObject List, then using contains to see if the Id is in the Set. Assuming it's a HashSet, those will be constant-time lookups. — jas, May 06 '15 at 07:51

score 1 · Answer 1 · answered May 06 '15 at 07:58

With Java 8 you can achieve this with a stream filter:

List<CustomObject> collect = existingValues.stream()
.filter(object -> ids.contains(object.getId()))
.collect(Collectors.toList());

To get familiar with java streams I recommand the offical Oracle Tutorial.

score 1 · Accepted Answer · answered May 11 '15 at 15:36

With Guava this is done as follows:

    final Set<Long> ids = ...; //initialize correctly

    List<CustomObject> results = FluentIterable.from(existingValues).filter(new Predicate<CustomObject>() {
        @Override
        public boolean apply(final CustomObject input) {
            return ids.contains(input.getId());
        }
    }).toList();

score 0 · Answer 3 · answered May 06 '15 at 09:09

Yes as @wuethrich44 says in Java 8 the solution is:

List<CustomObject> collect = existingValues.stream()
    .filter(object -> ids.contains(object.getId()))
    .collect(Collectors.toList());

And in the case you have a version older than Java 8:

List<CustomObject> collect = new ArrayList<CustomObject>();
for(CustomObject object: existingValues) {
    if(ids.contains(object.getId())) {
        collect.add(object);
    }
}

And for version older than Java 5:

List<CustomObject> collect = new ArrayList<CustomObject>();
Iterator<CustomObject> iterator = existingValues.iterator();
while(iterator.hasNext()) {
    CustomObject object = iterator.next();
    if(ids.contains(object.getId())) {
        collect.add(object);
    }
}

but the version with stream is better: faster in term of execution time, less verbose, and more readable if you are use to it

It's not always the case that the stream version is "faster in terms of execution time." — Louis Wasserman, May 07 '15 at 04:06

aburkov · Answer 4 · 2015-06-11T03:04:35.893

0

If you can use my xpresso library you can use list comprehensions with lambda expressions like this:

list<CustomObject> filtered = x.list(x.<CustomObject>yield().forEach(existingValues).when(x.lambdaP("x : f1(f0(x))",x.invoke("getId"),x.in(ids))));

edited Jun 11 '15 at 03:04

answered Jun 11 '15 at 02:40

aburkov

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Find intersection between collections with different objects

4 Answers4