Type problems with tuples & streams

Question

I have my own implementation for a few tuples, here's the gist of the code for pair:

public class Pair<A, B>  extends Tuple implements Serializable{
    ...
    public Pair(A a,B b){
        this.a=a;
        this.b=b;
    }
    ...
    public <T> Pair<T,B> mapA(Function<A,T> fun){
        return new Pair<T,B>(fun.apply(a),b);
    }
    ...
}

For some weird reason the following code does not work and the compiler seems to think that the resulting pair from the first mapping is <Object,String>.

List<Pair<String,String>> pairs = ...;
pairs
.stream()
.map(p->mapA(s->s.trim())
.map(p->mapA(s->s.toUpperCase()) //does not recognize a as a string
...

Could this be also Eclipse acting up? Running Eclipse Luna fwiw which seems to do a shitty job in general in determining generic types from functional interfaces.

EDIT: full example as requested

public class Pair<A, B> implements Serializable{
    public final A a;
    public final B b;

    public Pair(A a,B b) {
        this.a = a;
        this.b = b;
    }

    public <T> Pair<T,B> mapA(Function<A,T> fun){
        return new Pair<T,B>(fun.apply(a),b);
    }

}


List<Pair<String,String>> pairs = new ArrayList<>();
pairs.add(new Pair<String,String>("foo","bar"));
pairs.stream()
.map(p->p.mapA(s->s.trim()))
.map(p->p.mapA(s->s.toUpperCase()))
.collect(Collectors.toList());

Must be an eclipse issue ... no complains and errors in NetBeans ... did you try at least just compiling and running? — YoYo, Jul 02 '15 at 23:11
Version: Luna Service Release 2 (4.4.2) Build id: 20150219-0600 — Sotirios Delimanolis, Jul 02 '15 at 23:20
k, I'm one version behind with Version: Luna Service Release 1 (4.4.1)... I'll try installing Mars tomorrow and see if that helps — Seppo420, Jul 02 '15 at 23:23
Meh, having had to use Android Studio has kinda traumatized me for life. — Seppo420, Jul 02 '15 at 23:30

Bohemian · Answer 1 · 2015-07-03T03:15:14.230

4

Type inference for a typed method doesn't work with lambdas.

Also, you have a coding error (you were referencing the mapA() method without an instance).

You can fix the problem by explicitly typing the method:

pairs.stream()
.map(p -> p.mapA(s -> s.trim())) // input type is known here from stream
.map(p -> p.<String>mapA(s -> s.toUpperCase())) // but not here

Although only a style issue, you could rewrite the above using method references:

pairs.stream()
.map(p -> p.mapA(String::trim))
.map(p -> p.<String>mapA(String::toUpperCase))

edited Jul 03 '15 at 03:15

answered Jul 02 '15 at 23:30

Bohemian

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Works for the example but for some reason not for my actual use case. Triplet triplets ...; triplets.stream().mapC(MyItem::load).mapC(MyItem::getTitle) ... Complains that getTitle is not defined for object. i->i.getTitle() wont work either. – Seppo420 Jul 03 '15 at 07:42
I could obviously do something like mapC(r->MyItem.load(ref).getTitle()), but I'd kinda need to map the triplets to elsewhere anyways. – Seppo420 Jul 03 '15 at 07:43
Now the example is not working either anymore... darn eclipse... EDIT: turns out it works when using `p.mapA(String::trim)` but not with `p.mapA(s->s.trim())`... The real life case fails with both tho – Seppo420 Jul 03 '15 at 07:50
This one did the trick: `triplets.stream().map(t->t.mapC(MyItem::load).map(t->t.mapC(MyItem::getTitle))`... – Seppo420 Jul 03 '15 at 07:57

score 0 · Accepted Answer · answered Jul 08 '15 at 17:03

0

Updated to Eclipse Mars and the issue is completely gone.

answered Jul 08 '15 at 17:03

Seppo420

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Type problems with tuples & streams

2 Answers2