Want to create a stream of characters from char ar

问题:

From a char array, I want to construct a stream to use java 8 features such as filters and maps.

char[] list = {'a','c','e'};
Stream<Character> cStream = Stream.of(list);
// Stream<Character> cStream = Arrays.stream(list);

The first method does not work (Reason: change cStream to Stream<char[]>). The commented line does not also work (Reason: The method stream(T[]) in the type Arrays is not applicable for the arguments (char[])).

I know that if char[] list is changed to int[], everything works fine using IntStream. But I do not want to convert every char[] to int[] each time or change into a list when I need to use stream library on char array.

回答1:

You can use an IntStream to generate the indices followed by mapToObj:

char[] arr = {'a','c','e'};
Stream<Character> cStream = IntStream.range(0, arr.length).mapToObj(i -> arr[i]);

回答2:

A way to do this is via a String object:

char[] list = {'a','c','e'};
Stream<Character> charStream = new String(list).chars().mapToObj(i->(char)i);

I like to do it this way because all the complexity of transforming the array is wrapped into the String creation, and the wrapping of char is also performed behind the scene for me so I can focus on the business logic.

回答3:

A short and efficient way to create an IntStream from char[] array is to use java.nio.CharBuffer:

char[] list = {'a','c','e'};
IntStream stream = CharBuffer.wrap(list).chars();

This way you can use an IntStream interpreting the int values as characters. If you want a boxed Stream<Character> (which may be less efficient), use

Stream<Character> stream = CharBuffer.wrap(list).chars().mapToObj(ch -> (char)ch);

Using CharBuffer can be a little bit faster than IntStream.range as it has custom spliterator inside, so it does not have to execute an additional lambda (possibly as slow polymorphic call). Also it refers to the char[] array only once and not inside the lambda, so it can be used with non-final array variable or function return value (like CharBuffer.wrap(getCharArrayFromSomewhere()).chars()).

回答4:

The simplest change you can make to the code is change char[] to Character[].

Another way is to create a new ArrayList of the boxed chars:

char[] list = {'a','c','e'};
List<Character> listArray = new ArrayList<>();
for (char c : list)
    listArray.add(c);
Stream<Character> cStream = listArray.stream();

In addition, you can use Google Guava's Chars class, to replace the for loop with:

List<Character> listArray = Chars.asList(list);

回答5:

I believe the simplest way to convert to Character stream is

    char[] list = {'a','c','e'};

    Stream<Character> characters = Stream.ofAll(list);