Java stream - Get objects of consecutive same field

Question

I don't know how to describe the goal I want to achieve with a good title. I have a list of objects where each have a boolean member. I want to filter this list to get at the end the first objects from the list which all have this boolean member set to true.

This is how I would do it without streams, add every object to a new list for which the getter method returns true and stop when the first element returns false.

int i = 0;
while(i < list.size()){
    if(!list.get(i).isMemberTrue()){
        break;
    }
    else{
        newList.add(list.get(i));
    }
    i++;
}

Is this somehow possible with a sequential stream?

`list.stream().filter(member->member::isMemberTrue).findFirst()` — bananas, Sep 29 '16 at 06:38
one more thing if anyhow your first `member` from list is not member your loop will `break` so make sure what you are doing — bananas, Sep 29 '16 at 06:45
Yes the resulting list may be empty in case the first object returns already false. — Semaphor, Sep 29 '16 at 06:46
As you can see from the answers, you have found a case where switching to streams over the classic loop does *not* make sense. All solution attempts (so far) either use intermediate objects, give up potential parallelism, or iterate through the list multiple times. My advice is: stick to your loop. — mtj, Sep 29 '16 at 07:54

score 0 · Answer 1 · answered Sep 29 '16 at 06:37

0

Maybe something like this would work:

ArrayList<YourClass> newList = new ArrayList<>(
    list.stream()
   .filter(obj -> obj.isMemberTrue())
   .collect(Collectors.toList())
);

remember to import java.util.stream.Collectors if you want to return it as a list.

answered Sep 29 '16 at 06:37

Cicero

2,872
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I want to get the first consecutive objects for which it returns true not every object for which it returns true. – Semaphor Sep 29 '16 at 06:39
@semaphor check the comment :) – bananas Sep 29 '16 at 06:40
@Semaphor I don't know if that is possible with streams. – Cicero Sep 29 '16 at 06:42
just add `findFirst()` instead of `collect...` – bananas Sep 29 '16 at 06:43
@Asterisk Ninja But this will just return the first objec for which it returns true, but I want all objects from the beginning until the first returns false. – Semaphor Sep 29 '16 at 06:45
okey then let me edit – bananas Sep 29 '16 at 06:46

score 0 · Answer 2 · edited May 23 '17 at 12:00

0

you can use this code to get list

list.subList(0,list.indexOf(list.stream()
                    .filter(member->!member::isMemberTrue)
                    .findFirst()
                    .orElse(list.size()))

or you can use it as

int endIndex=list.indexOf(list.stream()
                    .filter(member->!member::isMemberTrue)
                    .findFirst()
                    .orElse(list.size());

then

list.sublist(0,endIndex);

In this I am getting new list by making use of List::sublist(..)and then I am usinglambda` to fetch all members before the condition goes false.

After being reviewed by Flown, I updated the answer Hope this helps! :)

edited May 23 '17 at 12:00

Community

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answered Sep 29 '16 at 07:02

bananas

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findFirst() will give you the object, not the index. So you'd have to actually use sublist(0, list.indexOf(firstObject)), provided that such a first object exists. – mtj Sep 29 '16 at 07:52
@mtj thanks for pointing out, I'll make the edit – bananas Sep 29 '16 at 07:59
1

@AsteriskNinja This should do your job: `int low = IntStream.range(0, list.size()).filter(i -> !list.get(i).isMemberTrue()).findFirst().orElse(0);`. But be careful, if the underlying implementation isn't a `RandomAccess` list. – Flown Sep 29 '16 at 08:08
@flown what if list passes `filter(..)` then your code will result `0` as low which is not needed instead one can use `orElse(list.size()`) – bananas Sep 29 '16 at 08:26
@AsteriskNinja you're right. It should be `.orElse(list.size());`. – Flown Sep 29 '16 at 09:05

score -1 · Answer 3 · answered Sep 29 '16 at 07:36

You can achieve the solution with streams. Just remember the predecessor in the list.

public class ListTest
{

    @Test
    public void test()
    {

        List<Foo> list = new ArrayList<>();
        list.add(new Foo(true));
        list.add(new Foo(true));
        list.add(new Foo(true));
        list.add(new Foo(false));
        list.add(new Foo(false));
        list.add(new Foo(true));
        list.add(new Foo(true));


        List<Foo> firstSequence = list.stream().filter(new Predicate<Foo>() {

            private boolean inFirstSequence = true;

            @Override
            public boolean test(Foo foo)
            {
                // turns into false, if first false value is found
                inFirstSequence = inFirstSequence && foo.b;

                return inFirstSequence;                
            }
        }).collect(Collectors.toList());

        Assert.assertEquals(3, firstSequence.size());
    }

    private class Foo
    {
        public Foo(boolean b)
        {
            super();
            this.b = b;
        }

        boolean b = true;
    }
}

Have you tried your code in parallel execution? Are you getting the same results? I guess not. So please don't use a state in a stateless expression since unexpected behavior can occur. — Flown, Sep 29 '16 at 07:44
@Flown. Thanks, for review. If i put the property to class ListTest I will agree with you. So the state is only used in the call stack of method test() ?!? — Dangermouse, Sep 29 '16 at 08:38
You can see an example why you shouldn't use state in `Stream` pipeline in the [documentation](https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html#Statelessness) — Flown, Sep 29 '16 at 09:03

Java stream - Get objects of consecutive same field

3 Answers3