import com.alibaba.fastjson.JSON;
import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class Main {
public static void main(String[] args) {
// 将Stream转换成List或Set
Stream<String> stream = Stream.of("I", "love", "you", "you", "too");
List<String> stringList = stream.collect(Collectors.toList()); // (1)
System.out.println(JSON.toJSONString(stringList));//["I","love","you","you","too"]
// Set<String> set = stream.collect(Collectors.toSet()); // (2)
// System.out.println(JSON.toJSONString(set));//["love","too","I","you"]
//返回的是一个String，不是List<String>
String string1 = stringList.stream().collect(Collectors.joining());
System.out.println(string1);//Iloveyouyoutoo
String string2 = stringList.stream().collect(Collectors.joining(","));
System.out.println(string2);//I,love,you,you,too
// public static Collector<CharSequence, ?, String> joining(CharSequence delimiter,
// CharSequence prefix,
// CharSequence suffix) {
String string3 = stringList.stream().collect(Collectors.joining(",", "{", "}"));
System.out.println(string3);//{I,love,you,you,too}
List<String> list = new ArrayList<>();
list.add("A");
list.add("B");
list.add("C");
list.add("C");
Set<String> stringSet = list.stream().collect(Collectors.toSet());
System.out.println(JSON.toJSONString(stringSet));//["A","B","C"]
//------------------------------------------------------------------------
Subject subject1 = new Subject(1, "语文", 91.0);
Subject subject2 = new Subject(2, "数学", 94.0);
Subject subject3 = new Subject(3, "英语", 92.0);
Subject subject4 = new Subject(4, "语文", 90.0);
List<Subject> subjectList = new ArrayList<>();
subjectList.add(subject1);
subjectList.add(subject2);
subjectList.add(subject3);
subjectList.add(subject4);
Map<Integer, String> map1 = subjectList.stream().collect(Collectors.toMap(Subject::getId, Subject::getName));
System.out.println(JSON.toJSONString(map1));//{1:"语文",2:"数学",3:"英语",4:"语文"}
// Map<String, Double> map2 = subjectList.stream().collect(Collectors.toMap(Subject::getName, Subject::getGrade));
// 出现重复key，报错：java.lang.IllegalStateException: Duplicate key 91.0
// System.out.println(JSON.toJSONString(map2));
Map<String, Double> map2 = subjectList.stream().collect(Collectors.toMap(Subject::getName, Subject::getGrade,
(oldValue, newValue) -> newValue));//方法引用只带方法名即可，不必带圆括号
//出现重复key则用新值覆盖
System.out.println(JSON.toJSONString(map2));//{"数学":94.0,"语文":90.0,"英语":92.0}
Map<String, Double> map3 = subjectList.stream().collect(Collectors.toMap(Subject::getName, Subject::getGrade,
(oldValue, newValue) -> oldValue));
//出现重复key继续用旧值
System.out.println(JSON.toJSONString(map3));//{"数学":94.0,"语文":91.0,"英语":92.0}
Map<Integer, Subject> map4 = subjectList.stream().collect(Collectors.toMap(Subject::getId,
Function.identity(), (oldValue, newValue) -> newValue));
System.out.println(JSON.toJSONString(map4));
//{1:{"grade":91.0,"id":1,"name":"语文"},2:{"grade":94.0,"id":2,"name":"数学"},3:{"grade":92.0,"id":3,"name":"英语"},4:{"grade":90.0,"id":4,"name":"语文"}}
//value是对象
Map<String, Subject> map5 = subjectList.stream().collect(Collectors.toMap(Subject::getName,
Function.identity(), (oldValue, newValue) -> oldValue, HashMap::new));
System.out.println(JSON.toJSONString(map5));
//{"数学":{"grade":94.0,"id":2,"name":"数学"},"语文":{"grade":91.0,"id":1,"name":"语文"},"英语":{"grade":92.0,"id":3,"name":"英语"}}
//这里的key必须是boolean型
//按照某个条件是否成立分为两组，value是list
Map<Boolean, List<Subject>> stringListMap1 = subjectList.stream().collect(Collectors.partitioningBy(s -> s.getName().equals("语文")));
System.out.println(JSON.toJSONString(stringListMap1));
//{false:[{"grade":94.0,"id":2,"name":"数学"},{"grade":92.0,"id":3,"name":"英语"}],true:[{"grade":91.0,"id":1,"name":"语文"},{"grade":90.0,"id":4,"name":"语文"}]}
//按照某个条件分为多组，value是list
Map<String, List<Subject>> stringListMap2 = subjectList.stream().collect(Collectors.groupingBy(Subject::getName));//这里只能是Subject::getName，如果写成subject.getName()则报错
System.out.println(JSON.toJSONString(stringListMap2));
//{"数学":[{"grade":94.0,"id":2,"name":"数学"}],"语文":[{"grade":91.0,"id":1,"name":"语文"},{"grade":90.0,"id":4,"name":"语文"}],"英语":[{"grade":92.0,"id":3,"name":"英语"}]}
//使用parallelStream是多管道处理，效率比stream高很多
Map<String, List<Subject>> stringListMap5 = subjectList.parallelStream().collect(Collectors.groupingBy(Subject::getName));//这里只能是Subject::getName，如果写成subject.getName()则报错
System.out.println(JSON.toJSONString(stringListMap5));
//{"数学":[{"grade":94.0,"id":2,"name":"数学"}],"语文":[{"grade":91.0,"id":1,"name":"语文"},{"grade":90.0,"id":4,"name":"语文"}],"英语":[{"grade":92.0,"id":3,"name":"英语"}]}
//按某个条件统计，每种key存在多少个总数
Map<String, Long> stringListMap3 = subjectList.stream().collect(Collectors.groupingBy(Subject::getName, Collectors.counting()));
System.out.println(JSON.toJSONString(stringListMap3));
//{"数学":1,"语文":2,"英语":1}
//按某个条件统计，key的某个属性的列表
Map<String, List<Double>> stringListMap4 = subjectList.stream().collect(Collectors.groupingBy(Subject::getName,
Collectors.mapping(Subject::getGrade, Collectors.toList())));
System.out.println(JSON.toJSONString(stringListMap4));
//{"数学":[94.0],"语文":[91.0,90.0],"英语":[92.0]}
//------------------------------------------------------------------------
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);
//串行输出
numbers.stream().forEach(System.out::print);//123456789
System.out.println();
//并行输出
numbers.parallelStream().forEach(System.out::print);//658973421
System.out.println();
//并行有序输出
numbers.parallelStream().forEachOrdered(System.out::print);//123456789
}
}
以上基于jdk1.8
参考：https://blog.csdn.net/Crystalqy/article/details/79310058 java Stream 和 parallelStream比较
https://blog.csdn.net/wangmuming/article/details/78518301 Java 8 – Convert List to Map（将 List 转换为 Map）
