Java 8 filter list of pojos based on nested object attribute

Question

I have a list of Thingy pojos, such that:

public class Thingy {
    private DifferentThingy nestedThingy;
    public DifferentThingy getNestedThingy() {
        return this.nestedThingy;
    }
}

...

public class DifferentThingy {
    private String attr;
    public String getAttr() {
        return this.attr;
    }
}

I want to filter a

List<Thingy>

to be unique based on the

attr

of the Thingy's

DifferentThingy

Here is what I have tried so far:

private List<Thingy> getUniqueBasedOnDifferentThingyAttr(List<Thingy> originalList) {
    List<Thingy> uniqueItems = new ArrayList<Thingy>();
    Set<String> encounteredNestedDiffThingyAttrs= new HashSet<String>();
    for (Thingy t: originalList) {
        String nestedDiffThingyAttr = t.getNestedThingy().getAttr();
        if(!encounteredNestedDiffThingyAttrs.contains(nestedDiffThingyAttr)) {
            encounteredNestedDiffThingyAttrs.add(nestedDiffThingyAttr);
            uniqueItems.add(t);
        }
    }
    return uniqueItems;
}

I want to do this using a Java 8 stream and lambdas for the two getters that end up retrieving the attribute used for determining uniqueness, but am unsure how. I know how to do it when the attribute for comparison is on the top level of the pojo, but not when the attribute is nested in another object.

Possible duplicate of Java Lambda Stream Distinct() on arbitrary key? — castletheperson
– castletheperson, Commented Feb 20, 2017 at 22:36

Divers · Accepted Answer · 2017-02-20 22:00:52Z

4

You can do it that way:

originalList.stream()
    .collect(Collectors.toMap(thing -> thing.getNestedThingy().getAttr(), p -> p, (p, q) -> p))
    .values();

Not sure if the most optimal way though, I'm on Android, so don't use streams in daily job.

UPD

For someone can not compile it, there is full code of my test file.

edited Feb 20, 2017 at 22:00

answered Feb 20, 2017 at 21:12

Divers

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8 Comments

Grzegorz Górkiewicz Over a year ago

Anyone tested the above? I have got a java.lang.ClassCastException: java.util.HashMap$Values cannot be cast to java.util.List.

Andreas Over a year ago

My solution is about the same: public static Collection<Thingy> getUniqueBasedOnDifferentThingyAttr(List<Thingy> originalList) { return originalList.stream() .collect(Collectors.toMap(t -> t.getNestedThingy().getAttr(), t -> t)) .values(); }

ChiefTwoPencils Over a year ago

@Andreas, I think it depends on which one they want when a collision happens.

Divers Over a year ago

@GrzegorzGórkiewicz yes, I wrote it in idea, and it's 100% working code

Divers Over a year ago

@JellyRaptor I've added link to full code of my test class.

|

Mathias Dpunkt · Accepted Answer · 2017-02-20 21:35:40Z

4

What about making it far more explicit using javaslang

javaslang.collection.List.ofAll(thingies)
  .distinctBy(t -> t.getNestedThingy().getAttr());

Javaslang core is a functional library for Java 8+. It helps to reduce the amount of code and to increase the robustness. A first step towards functional programming is to start thinking in immutable values. Javaslang provides immutable collections and the necessary functions and control structures to operate on these values. The results are beautiful and just work.

answered Feb 20, 2017 at 21:35

Mathias Dpunkt

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Comments

Grzegorz Górkiewicz · Accepted Answer · 2017-02-20 22:48:17Z

2

I keep a set of unique nested thingies' attributes... then I try to add an element to it. When the set "grows", it means that an unique element has been encountered. When the set size does not change, it means that this Thingy has a NestedThingy with an already encountered attribute. Together with a simple test that would be:

public class Thingies {

    public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
        final Set<Object> seen = Collections.synchronizedSet(new HashSet<>());
        return t -> seen.add(keyExtractor.apply(t));
    }

    private static List<Thingy> getUniqueBasedOnDifferentThingyAttr(List<Thingy> originalList) {
        return originalList.stream()
                .filter(distinctByKey(thingy -> thingy.getNestedThingy().getAttr()))
                .collect(toList());
    }

    public static void main(String[] args) {
        List<Thingy> originalList = new ArrayList<>();
        originalList.add(new Thingy(new DifferentThingy("first")));
        originalList.add(new Thingy(new DifferentThingy("first")));
        originalList.add(new Thingy(new DifferentThingy("second")));
        System.out.println(getUniqueBasedOnDifferentThingyAttr(originalList));
    }
}

Output:

[first, second]

The original answer, which helped the OP, (a bit lengthy and not good for parallel streams) contained such a method instead:

private static List<Thingy> getUniqueBasedOnDifferentThingyAttr(List<Thingy> originalList) {
    final Set<String> uniqueAttributes = new HashSet<>(originalList.size());
    return originalList.stream()
            .filter(thingy -> {
                int initialSize = uniqueAttributes.size();
                uniqueAttributes.add(thingy.getNestedThingy().getAttr());
                return initialSize != uniqueAttributes.size();
            }).collect(toList());
}

edited Feb 20, 2017 at 22:48

answered Feb 20, 2017 at 21:23

Grzegorz Górkiewicz

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10 Comments

JellyRaptor Over a year ago

This works if I change toList() to Collectors.toList()

Grzegorz Górkiewicz Over a year ago

Then go for it ;) I can try to make it a bit shorter with an distinctBy method implementation.

Divers Over a year ago

Change outer value from inside stream chain looks duty for me.

Grzegorz Górkiewicz Over a year ago

@Divers, it is contained in one method. Reassigning a variable is a bad practice (here impossible because of final), making use of its mutability (here: add method call) is a common practice, especially in void methods. Having said this, I still like your code... it returns an instance of Collection<Thingy> instead of List<Thingy>, which is its only drawback (very easy to fix).

Grzegorz Górkiewicz Over a year ago

Thank you all for your constructive criticism, I will edit my answer.

|

Hank D · Accepted Answer · 2017-02-21 15:53:09Z

2

You might use a TreeSet whose comparator reaches into the subobject's attribute:

private static List<Thingy> getUniqueBasedOnDifferentThingyAttr(List<Thingy> originalList) {

    Set<Thingy> uniqueSet = new TreeSet<Thingy>(Comparator.comparing(thingy -> thingy.getNestedThingy().getAttr()));

    return originalList.stream()
            .filter(uniqueSet::add)
            .collect(Collectors.toList());
}

uniqueSet::add makes an adequate uniqueness filter because it returns true only if the item is not already in the set.

answered Feb 21, 2017 at 15:53

Hank D

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2 Comments

JellyRaptor Over a year ago

Using an original list of length 5 with 3 "unique" items, this solution was, on average, 6 times faster (11.8ms to 66ms) than the originally accepted answer by @GrzegorzGórkiewicz. I like the use of Comparator as well.

Grzegorz Górkiewicz Over a year ago

Imho Hank D's answer should be the accepted one. It's simple and somehow beautiful.

user_3380739 · Accepted Answer · 2023-01-08 04:11:11Z

0

With StreamEx:

StreamEx.of(originalList).distinct(e -> e.getNestedThingy().getAttr()).toList();

Or by abacus-common

Stream.of(originalList).distinct(e -> e.getNestedThingy().getAttr()).toList();

edited Jan 8, 2023 at 4:11

answered Feb 21, 2017 at 19:24

user_3380739

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