Java Stream flatMapToLong()示例

Stream flatMapToLong(Function mapper) 将给定的mapper函数应用于流的每个元素，并返回一个LongStream。Stream flatMapToLong(Function mapper)是一个 中间操作。 中间操作在一个Stream实例上被调用，在它们完成处理后，它们给出一个Stream实例作为输出:

注意： 每个映射的流在其内容被放入该流后被关闭。如果一个被映射的流是空的，则使用一个空流。

语法

LongStream flatMapToLong(Function <? super T, ? extends LongStream> mapper)

其中，LongStream是一串原始的 长值元素的序列，T是流元素的类型。流元素的类型。映射器是一个无状态函数
它被应用于每个元素，并且该函数 返回新的流。

例1： flatMapToLong()的操作是将字符串解析为长字符串。

// Java code for Stream flatMapToLong// (Function mapper) to get an LongStream// consisting of the results of replacing// each element of this stream with the// contents of a mapped stream.import java.util.*;import java.util.stream.LongStream;  class GFG {      // Driver code    public static void main(String[] args)    {          // Creating a list of Strings        List<String> list = Arrays.asList("1", "2", "3",                                          "4", "5");          // Using Stream flatMapToLong(Function mapper)        list.stream().flatMapToLong(num ->                      LongStream.of(Long.parseLong(num))).                            forEach(System.out::println);    }}

输出:

12345

例2： flatMapToLong()的操作是用字符串的长度来映射字符串。

// Java code for Stream flatMapToLong// (Function mapper) to get an LongStream// consisting of the results of replacing// each element of this stream with the// contents of a mapped stream.import java.util.*;import java.util.stream.LongStream;  class GFG {      // Driver code    public static void main(String[] args)    {          // Creating a List of Strings        List<String> list = Arrays.asList("Geeks", "GFG",                                 "GeeksforGeeks", "gfg");          // Using Stream flatMapToLong(Function mapper)        // to get length of all strings present in list        list.stream().flatMapToLong(str ->                         LongStream.of(str.length())).                        forEach(System.out::println);    }}

输出:

53133

例3： flatMapToLong()函数有返回流的操作。

// Java code for Stream mapToLong// (ToLongFunction mapper) to get a// LongStream by applying the given function// to the elements of this stream.import java.util.*;import java.util.stream.LongStream;  class GFG {      // Driver code    public static void main(String[] args)    {        // Creating a list of Strings        List<String> list = Arrays.asList("5.36","27.9",                                         "3","4.2","5");          // Using Stream mapToLong(ToLongFunction mapper)        // and displaying the corresponding LongStream        list.stream().flatMapToLong(n                     -> LongStream.of(Long.parseLong(n)) )                     .forEach(System.out::println);    }}

输出:

Exception in thread "main" java.lang.NumberFormatException: For input string: "5.36"

注意： Java NumberFormatException 通常发生在你试图将一个字符串转换为一个数字值的时候，比如int、float、double、long等等。