在Java 8中,Stream.map()和Stream.flatMap()方法之间有什么区别?


当前回答

.map用于A -> B映射

Stream.of("dog", "cat")              // stream of 2 Strings
    .map(s -> s.length())            // stream of 2 Integers: [3, 3]

它将任意项A转换为任意项b


.flatMap用于A ->流< B>连接

Stream.of("dog", "cat")             // stream of 2 Strings
    .flatMapToInt(s -> s.chars())   // stream of 6 ints:      [d, o, g, c, a, t]

it——1将任何项A转换为Stream< B>,然后——2将所有流连接到一个(平面)流。Javadoc


注1:虽然后面的例子是一个原语流(IntStream),而不是一个对象流(stream),但它仍然说明了. flatmap的思想。

注意2:尽管有这个名字,String.chars()方法返回的是整数。所以实际的集合将是:[100,111,103,99,97,116] ,其中100是“d”的代码,111是“o”的代码,等等。同样,为了说明目的,它被表示为[d, o, g, c, a, t]。

其他回答

流。flatMap,顾名思义,是映射和平面操作的组合。这意味着您首先将一个函数应用到元素上,然后将其压平。流。Map仅将函数应用于流,而不会将流平展。

为了理解流的扁平化是由什么组成的,考虑一个像[[1,2,3],[4,5,6],[7,8,9]]这样的结构,它有“两个层次”。扁平化意味着将其转换为“一级”结构:[1,2,3,4,5,6,7,8,9]。

map()和flatMap()

map ()

只接受一个函数<T, R>一个lambda参数,其中T是元素,R是使用T构建的返回元素。最后,我们将有一个带有类型为R的对象的流。一个简单的例子可以是:

Stream
  .of(1,2,3,4,5)
  .map(myInt -> "preFix_"+myInt)
  .forEach(System.out::println);

它只是取Type Integer的元素1到5,使用每个元素从String类型中构建一个值为“prefix_”+integer_value的新元素,并打印出来。

flatMap ()

知道flatMap()接受一个函数F<T, R> where是很有用的

T is a type from which a Stream can be built from/with. It can be a List (T.stream()), an array (Arrays.stream(someArray)), etc.. anything that from which a Stream can be with/or form. in the example below each dev has many languages, so dev. Languages is a List and will use a lambda parameter. R is the resulting Stream that will be built using T. Knowing that we have many instances of T, we will naturally have many Streams from R. All these Streams from Type R will now be combined into one single 'flat' Stream from Type R.

例子

Bachiri Taoufiq的例子(见这里的答案)1简单易懂。为了清晰起见,假设我们有一个开发团队:

dev_team = {dev_1,dev_2,dev_3}

,每个开发人员都懂多种语言:

dev_1 = {lang_a,lang_b,lang_c},
dev_2 = {lang_d},
dev_3 = {lang_e,lang_f}

在dev_team上应用Stream.map()来获取每个开发人员的语言:

dev_team.map(dev -> dev.getLanguages())

会给你这样的结构:

{ 
  {lang_a,lang_b,lang_c},
  {lang_d},
  {lang_e,lang_f}
}

它基本上是一个List<List<Languages>> /Object[Languages[]]。不是很漂亮,也不像java8 !!

使用Stream.flatMap(),你可以“扁平化”的东西,因为它采取上述结构 并将其转换为{lang_a, lang_b, lang_c, lang_d, lang_e, lang_f},基本上可以作为List<Languages>/Language[]/etc…

所以最后,你的代码会像这样更有意义:

dev_team
   .stream()    /* {dev_1,dev_2,dev_3} */
   .map(dev -> dev.getLanguages()) /* {{lang_a,...,lang_c},{lang_d}{lang_e,lang_f}}} */
   .flatMap(languages ->  languages.stream()) /* {lang_a,...,lang_d, lang_e, lang_f} */
   .doWhateverWithYourNewStreamHere();

或者仅仅是:

dev_team
       .stream()    /* {dev_1,dev_2,dev_3} */
       .flatMap(dev -> dev.getLanguages().stream()) /* {lang_a,...,lang_d, lang_e, lang_f} */
       .doWhateverWithYourNewStreamHere();

何时使用map()和flatMap():

Use map() when each element of type T from your stream is supposed to be mapped/transformed to a single element of type R. The result is a mapping of type (1 start element -> 1 end element) and new stream of elements of type R is returned. Use flatMap() when each element of type T from your stream is supposed to mapped/transformed to a Collections of elements of type R. The result is a mapping of type (1 start element -> n end elements). These Collections are then merged (or flattened) to a new stream of elements of type R. This is useful for example to represent nested loops.

Java 8 之前:

List<Foo> myFoos = new ArrayList<Foo>();
    for(Foo foo: myFoos){
        for(Bar bar:  foo.getMyBars()){
            System.out.println(bar.getMyName());
        }
    }

后Java 8

myFoos
    .stream()
    .flatMap(foo -> foo.getMyBars().stream())
    .forEach(bar -> System.out.println(bar.getMyName()));

我有一种感觉,这里的大多数答案都把简单的问题复杂化了。如果你已经理解了地图是如何工作的,那就很容易掌握了。

在使用map()时,有些情况下我们可能会得到不需要的嵌套结构,flatMap()方法的设计是通过避免换行来克服这一问题。


例子:

1

List<List<Integer>> result = Stream.of(Arrays.asList(1), Arrays.asList(2, 3))
  .collect(Collectors.toList());

我们可以使用flatMap来避免使用嵌套列表:

List<Integer> result = Stream.of(Arrays.asList(1), Arrays.asList(2, 3))
  .flatMap(i -> i.stream())
  .collect(Collectors.toList());

2

Optional<Optional<String>> result = Optional.of(42)
      .map(id -> findById(id));

Optional<String> result = Optional.of(42)
      .flatMap(id -> findById(id));

地点:

private Optional<String> findById(Integer id)

flatMap()还利用了流的部分延迟求值。它将读取第一个流,只有在需要时才会进入下一个流。这里详细解释了这种行为:flatMap保证是懒惰的吗?

map() takes a Stream and transform it to another Stream. It applies a function on each element of Stream and store return value into new Stream. It does not flatten the stream. But flatMap() is the combination of a map and a flat operation i.e, it applies a function to elements as well as flatten them. 2) map() is used for transformation only, but flatMap() is used for both transformation and flattening. please read more here. https://javaint4bytes.blogspot.com/2022/11/stream-flatmap-in-java-with-examples.html