Java 8 List to Map with stream (avoid duplicates)

Question

I have two classes:

public class Adegae implements java.io.Serializable {
    private AdegaeId id;
    ...
}

and

public class AdegaeId implements java.io.Serializable {

    private String gpi;
    private String mccode;
    private String aecode;
    ...
}

I also have a list that have elements with duplaicates of aecode:

List<Adegae> adegaes = ...

I'm building my map like so:

Map<String, Adegae> adegaeMap = adegaes.stream()
            .collect(Collectors.toMap(adegae -> adegae.getId().getAecode().trim(), 
                    Function.identity()));

When I run my code I get this exception:

java.lang.IllegalStateException: Duplicate key my.package.Adegae@678e3d87
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
at java.util.HashMap.merge(HashMap.java:1245)
at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)

I wonder if there is a way to collect to a Map by dropping the duplicates at the same time.

Yes, you need to supply a mergeFunction as the 3rd parameter. See the linked question. — Tunaki, Apr 28 '16 at 15:48
You would change adegaes from List to Set (Set avoid duplicates), can do something like : List list = ... Set set = new HashSet(list); — Daniel Hernández, Apr 28 '16 at 15:55
@DanielHernández Trickier since the duplicates aren't the object themselves but a property of the objects. — Tunaki, Apr 28 '16 at 16:03
Does `Adegae` override the `equals()` method to use their `id` to test equality? If so, this is a slightly different question with a very different answer. Basing equality on the equivalence of ID is a common approach for persistent entities. — erickson, Apr 28 '16 at 17:59
@erickson Why is it different? `equals` doesn't come into play here. The `mergeFunction` is used when elements are mapped to the same key, which in this case is the id. `equals` is unused. The linked question answers exactly that (`toMap` collector for both) — Tunaki, Apr 28 '16 at 18:52
@Tunaki It's different because using `distinct()` , if applicable, expresses the intent more clearly and succinctly than a merge operator. Applicability depends on the notion of equality, so `equals()` does come in to play. — erickson, Apr 28 '16 at 18:58
@erickson Indeed it does come into play for `distinct()` and you're right that it would make sense to have an `equals` comparing the id. — Tunaki, Apr 28 '16 at 18:59
In the other question: while it's likely `Person` is an entity and *could* be tested by ID, it seems more about merging distinct entities by a business key, like `name`. — erickson, Apr 28 '16 at 19:01

score 0 · Answer 1 · answered Apr 28 '16 at 15:48

0

You can use the distinct() method on Stream to eliminate the duplicates before further processing.

answered Apr 28 '16 at 15:48

erickson

265,237
58
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Trickier since the duplicates aren't the object themselves but a property of the objects. – Tunaki Apr 28 '16 at 16:03
Yes, it depends on the `equals()` method, but here, one would hope that objects with the same ID are equal. – erickson Apr 28 '16 at 17:51
1

This solution cant apply to my case since this is auto-generated class from Hibernate and I'm using only portion of the whole key. – serge Apr 29 '16 at 14:26

score 0 · Accepted Answer · answered Apr 28 '16 at 20:41

0

Map<String, Adegae> adegaeMap = adegaes.stream() .collect(Collectors.toMap(adegae -> adegae.getId().getAecode().trim(), Function.identity(), (e1, e2) -> e1)); may help in your case

answered Apr 28 '16 at 20:41

Andrei_N

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2
15

That's exactly what Tunaki's answer is in the duplicate question – serge Apr 29 '16 at 14:23

Java 8 List to Map with stream (avoid duplicates)

2 Answers2