Java 8 Stream unique characters from array

问题

Trying to write a simple program that will print the unique words from an input array in Java8 . For example if the input is

   String[] input = {"This", "is", "This", "not"};

The program should output [T, h, i, s, n, o, t] , the order of elements should follow the same pattern as they appear in input. My approach is to split the input , then map, distinct and finally collect it toList. But the following code is printing list of streams instead of words, what am i missing?.E.g.

String[] input = {"This", "is", "This", "not"};
       System.out.println(Arrays.stream(input)
               .map(word -> word.split(""))
               .map(Arrays::stream)
               .distinct()
               .collect(toList()));

Current output

[java.util.stream.ReferencePipeline$Head@548c4f57, java.util.stream.ReferencePipeline$Head@1218025c, java.util.stream.ReferencePipeline$Head@816f27d, java.util.stream.ReferencePipeline$Head@87aac27]

I am interested to see if there are some other ways in Java8 to achieve the same.

回答1:

You need to use flatMap in order to map with the content of the stream . As you noticed that Arrays::stream generate separate streams and what you want is to flatten all those streams into single stream E.g.

   System.out.println(Stream.of(input)
           .map(w -> w.split(""))
           .flatMap(Arrays::stream)
           .distinct()
           .collect(Collectors.toList()));

Output

[T, h, i, s, n, o, t]

回答2:

Another solution to complement @sol4me's answer:

System.out.println(Stream.of(input).flatMapToInt(CharSequence::chars)
    .distinct().mapToObj(c -> Character.valueOf((char) c))
    .collect(Collectors.toList()));

Could probably be written in a more efficient manner.

The JDK doesn't define CharStream, unfortunately...

回答3:

Java in Action 8 : has nice explanation of using flat map, I am reiterate the solution which is posted earlier and giving the explanation as per the book.

String[] input = {"This", "is", "This", "not"};

map function returns a stream consisting of the results of applying the given function to the elements of this stream.

<R> Stream<R> map(Function<? super T, ? extends R> mapper);

    List<Stream<String>> steramsOfStream = Arrays.stream(input)
            .map(word -> word.split(""))
            .map(Arrays::stream)
            .distinct().collect(Collectors.toList());

    steramsOfStream.forEach(a -> {
            a.forEach(System.out::print);
            System.out.print("\n");
        });

This will produce the following output

output : [This,is, This, not ]

flat map Returns a stream consisting of the results of " replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.

<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);

    List<String> listOfStringss = Arrays.stream(input)
            .map(word -> word.split(""))
            .flatMap(Arrays::stream)
            .distinct().collect(Collectors.toList());

    System.out.println(listOfStringss);

produces output : [T, h, i, s, n, o, t] [

来源：https://stackoverflow.com/questions/27881627/java-8-stream-unique-characters-from-array

标签

java

arrays

functional-programming