passing lambda as method arg and sorting map

Question

i must create Sorted method, called with map arguments and lambda-expression. The method returns a sorted version of any map passed as the first argument, and the sort order is determined by the lambda expression, given as the second argument. i created something like that(dont work correctly):

public Map sorted(Map map, Function<Set> funct){


    System.out.println(map.entrySet()
       .stream()
       .sorted((Comparator) funct)
       .collect(Collectors.toList()));
     return null;
}

Any ideas ?
Thanks for help ;)

You are supposed to pass a Comparator<Map.Entry<KeyType,ValueType>> to the sorted method. — Eran
– Eran, Commented Apr 25, 2017 at 9:51
You "must"? Because it's much more readable to just Map<K,V> sorted = new TreeMap<>(comparator); sorted.putAll(map); return sorted; — Olivier Grégoire
– Olivier Grégoire, Commented Apr 25, 2017 at 10:37

Eugene · Accepted Answer · 2017-04-25 10:06:37Z

3

If you want a sorted map that would be a TreeMap and assuming the comparator sorts by Key, it could look like this:

 public static <K, V> TreeMap<K, V> sorted(Map<K, V> map, Comparator<? super K> cmp) {

    return map.entrySet()
            .stream()
            .collect(Collectors.toMap(Map.Entry::getKey,     
                          Map.Entry::getValue, 
                          (left, right) -> left, 
                          () -> new TreeMap<K, V>(cmp)));

}

And an invocation of it would look like this for example:

 System.out.println(sorted(map, Comparator.naturalOrder()));

answered Apr 25, 2017 at 10:06

Eugene

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1 Comment

Łukasz Senderowski Over a year ago

Thank you for help, it was very usefull ;)

Robin Topper · Accepted Answer · 2017-04-25 12:09:54Z

2

When I read the question I was thinking of a more general solution where you can sort the map on either keys or values. In that case the solution looks like this:

public static <K,V> Map<K,V> sorted(Map<K,V> map, Comparator<Map.Entry<K, V>> comparator){
    return map.entrySet()
       .stream()
       .sorted(comparator)
       .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
}

If you want to want to sort by value, you can use this method like this:

sorted(yourMap, Entry.comparingByValue());

edited Apr 25, 2017 at 12:09

answered Apr 25, 2017 at 11:17

Robin Topper

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3 Comments

Olivier Grégoire Over a year ago

The usage example can be "shortened" as sorted(map, Comparator.comparing(Map.Entry::getValue)).

Robin Topper Over a year ago

And even shorter with sorted(map, Entry.comparingByValue())

Olivier Grégoire Over a year ago

Live and learn, they say! :) Realistically, I'd use the appropriate usage directly in the answer, instead of showing a somewhat complicated construct.

holi-java · Accepted Answer · 2017-04-25 11:15:21Z

-1

Function declared 2 generic arguments: T and R, but you only given 1 generic parameter Set.
collecting a stream to a Map you can using Collectors.toMap rather than Collectors.toList which is collecting a stream to a List.
Comparator is more like a BiFunction/ToIntBiFunction due to the number of generic arguments of a Functional Interface includes parameters & return type.

IF you want to refer a ToIntBiFunction to a Comparator you can use this:

void sortByEntry() {
    Map<?, ?> map = new HashMap<>();
    Map<?, ?> sorted = sorted(map, this::comparingEntry);
}

<K, V> Map<K, V> sorted(Map<K, V> map, 
                        ToIntBiFunction<Entry<K, V>, Entry<K, V>> comparator) {
    return map.entrySet().stream()
            .sorted(comparator::applyAsInt)
            .collect(Collectors.toMap(Entry::getKey, Entry::getValue
                                    , (v1, v2) -> v2, LinkedHashMap::new));
}

<K, V> int comparingEntry(Entry<K, V> left, Entry<K, V> right) {
    return ...;
}

answered Apr 25, 2017 at 11:15

holi-java

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4 Comments

Olivier Grégoire Over a year ago

Comparator is a @FunctionalInterface. There is therefore no need to replace it with ToIntBiFunction. Doing so adds complexity where there is none. This is why I downvoted this answer. If we remove that ToIntBiFunction, the whole answer collapses and falls back on to @Eugene's or @RobinTopper' answer, making it a duplicate.

holi-java Over a year ago

@OlivierGrégoire first, thanks for your comment. I know Comparator is a Functional Interface, but I want to tell how to alternative between Functional Interfaces to OP. and RobinTopper's answer is after mine.

Olivier Grégoire Over a year ago

The intent of your answer isn't really clear then. Would you mind edit your answer to make that more prominent? This way I can retract my downvote as well (since it's locked until further edit).

holi-java Over a year ago

@OlivierGrégoire It doesn't matter, I don't matter the votes. but still thanks for your feedback again.

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