How to change HashMap to LinkedHashMap via Stream

Question

I'm new in Java and now I try lambda/stream.

How can I sort the Map by value?

I tried as follows, but it's not correct:

time = time.entrySet().stream()
           .sorted(Comparator.comparing(Map.Entry::getValue))
           .collect(Collectors.toMap((String)Map.Entry::getKey, 
                      (Long)Map.Entry::getValue, LinkedHashMap::new));

I found the following method, but don't know how to use it:

Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,   
                        Function<? super T, ? extends U> valueMapper,   
                        BinaryOperator<U> mergeFunction,   
                        Supplier<M> mapSupplier)

score 0 · Accepted Answer · answered Dec 10 '19 at 11:42

0

You forgot to specify the merge function.

time = time.entrySet()
           .stream()
           .sorted(Comparator.comparing(Map.Entry::getValue))
           .collect(Collectors.toMap(Map.Entry::getKey, 
                                    Map.Entry::getValue,
                                    (v1,v2)->v1,
                                    LinkedHashMap::new));

BTW, assuming that time is not a raw Map, you don't have to cast the key to String and the value to Long. And trying to cast the method references Map.Entry::getKey and Map.Entry::getValue is wrong anyway, since the method references implement the Function interface, so they shouldn't be cast to String or Long if you want your code to pass compilation.

answered Dec 10 '19 at 11:42

Eran

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Great thanks! I see it work. By can you help to understand- what (v1,v2)-> v1 means? It means that if v1 and v2 has the same value, first one in map will be v1, right? – Khilarian Dec 10 '19 at 11:53
@Khilarian it means that if the Stream maps two values to the same key, it will use that merge function, which will take the 2 values and return the first value. Since your Stream source is a Map, there will be no values to merge, so you can pass any merge function here and it won't affect the result. – Eran Dec 10 '19 at 11:56
thanks ) it's easier to understand that I read earlier)) – Khilarian Dec 10 '19 at 12:03

How to change HashMap to LinkedHashMap via Stream

1 Answers1