浅谈java8 stream flatMap流的扁平化操作

 @Test
 public void flapMapTest() {
 List<Integer> list = (List<Integer>) Stream.of(Arrays.asList(1, 2, 3, 4, 5, 6), Arrays.asList(8, 9, 10, 11, 12))
  .flatMap(test -> test.stream()).collect(Collectors.toList());
 
 for (int i = 0, length = list.size(); i < length; i++) {
  System.out.println(list.get(i));
 }
 
 }

String[] words = new String[]{"Hello","World"}; 
List<String[]> a = Arrays.stream(words)
 
.map(word -> word.split("")) 
.distinct() 
.collect(toList()); 
a.forEach(System.out::print);

String[] words = new String[]{"Hello","World"}; 
List<String> a = Arrays.stream(words)
 
.map(word -> word.split("")) 
.flatMap(Arrays::stream) 
.distinct() 
.collect(toList()); 
a.forEach(System.out::print);

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
 
public class FlatMap {
  public static void main(String[] args) {
    //扁平化流
    //找出数组中唯一的字符
    String[] strArray = {"hello", "world"};
 
    //具体实现
    List<String> res = Arrays.stream(strArray)
        .map(w -> w.split(""))
        .flatMap(Arrays::stream)
        .distinct()
        .collect(Collectors.toList());
    System.out.println(res);
 
    //TODO 案例
    System.out.println("--------------------------------");
    //Demo1:给定数组，返回数组平方和(直接使用映射)
    //[1,2,3,4]=>[1,4,9,16]
    Integer[] nums1 = {1, 2, 3, 4};
    List<Integer> nums1List = Arrays.asList(nums1);
    List<Integer> res1 = nums1List.stream().map(i -> i * i).collect(Collectors.toList());
    System.out.println(res1);
 
    System.out.println("--------------------------------");
    //Demo2:给定两数组，返回数组对
    //[1,2,3],[3,4]=>[1,3],[1,4],[2,3],[2,4],[3,3],[3,4]
    Integer[] nums2 = {1, 2, 3};
    Integer[] nums3 = {3, 4};
    List<Integer> nums2List = Arrays.asList(nums2);
    List<Integer> nums3List = Arrays.asList(nums3);
 
    //使用2个map嵌套过滤
    List<int[]> res2 = nums2List.stream().flatMap(i -> nums3List.stream().map(j -> new int[]{i, j})).collect(Collectors.toList());
    System.out.println(res2.size());
 
    System.out.println("--------------------------------");
    //Demo3:针对Demo2和Demo1组合返回总和能被3整除的数对
    //(2,4)和(3,3)是满足条件的
    List<int[]> res3 = nums2List.stream().flatMap(i -> nums3List.stream().filter(j -> (i + j) % 3 == 0).map(j -> new int[]{i, j})).collect(Collectors.toList());
    System.out.println(res3.size()); 
  }
}

List<String> output = wordList.stream().
map(String::toUpperCase).
collect(Collectors.toList());

List<Integer> nums = Arrays.asList(1, 2, 3, 4);
List<Integer> squareNums = nums.stream().
map(n -> n * n).
collect(Collectors.toList());

Stream<List<Integer>> inputStream = Stream.of(
 Arrays.asList(1),
 Arrays.asList(2, 3),
 Arrays.asList(4, 5, 6)
 );
Stream<Integer> outputStream = inputStream.
flatMap((childList) -> childList.stream());