java8之数据流Stream的Map和flatMap的区别

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package com.lyzx.day03; import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.Stream; import org.junit.Test; /**  * 复习一下Map和flatMap  * 关于Map和flatMap的区别  * / public class T1 {     private static List  words = Arrays.asList( "hello world",                                                         "hello java",                                                         "hello hello");     private static List  wordList = Arrays.asList( "aa", "bb", "cc");          @Test     public void test1(){         words.stream()               //对words的每一项都做切割操作，把每一个字符串都转换为数组               //执行完后数据结构如下{[ "hello", "world"],[ "hello", "java"],[ "hello", "hello"]}              .map(item - > item.split( " "))               //对每个数组做单独的遍历操作              .forEach(arr- >{                  for(String item : arr){                      System.out.println(item);                  }              });     }          @Test     public void test2(){         words.stream()               //把每一项转化为数组包含列表的Stream流               //然后这个函数把所有的List里面的字符串都取出来放在了一个集合中，这个集合做下一次执行的数据源               //{ "hello", "world", "hello", "java", "hello", "hello"}              .flatMap(item- >{                  String[] arr = item.split( " ");                  return Arrays.asList(arr).stream();              })               //根据每一项的HashCode和equals方法做去重操作              .distinct()               //打印每一项              .forEach(item- >System.out.println(item));     }          @Test     public void test3(){         wordList.stream()                  //把T1的getCharacter方法应用于wordList的每一项,wordList每一项都是String                  //getCharacter方法的参数也是String，符合函数式接口                  //通过map方法就把wordList的每一项都转换为了存放单个字符(以字符串的形式存放)列表的Stream流即Stream <List >                 .map(T1::getCharacter)                  //对转换后的每一项(List)分别做遍历操作                 .forEach(sm- >{                     sm.forEach(System.out::println);                 });     }          @Test     public void test4(){         wordList.stream()                 .flatMap(T1::getCharacter)                 .distinct()                 .forEach(System.out::println);     }          public static Stream  getCharacter(String str){         char[] array = str.toCharArray();         List  list = new ArrayList ();                  for(char item : array){             list.add(String.valueOf(item));         }         Stream  stream = list.stream();         return stream;     } }