1 package com.example.mass_study.test01.anything;
2
3 import java.util.ArrayList;
4 import java.util.List;
5 import java.util.Map;
6 import java.util.concurrent.ConcurrentHashMap;
7 import java.util.function.Function;
8 import java.util.function.Predicate;
9 import java.util.stream.Collectors;
10
11 /**
12 * @description:
13 * @author: luguilin
14 * @date: 2022-04-13 11:17
15 **/
16
17
18 public class StreamToMapTest {
19 public static void main(String[] args) {
20 List<User> users = new ArrayList<>();
21 User user1 = new User(1, "1");
22 User user2 = new User(2, "2");
23 User user3 = new User(3, "3");
24 users.add(user1);
25 users.add(user2);
26 users.add(user3);
27 // 将id属性读取出来组成list
28 List<Integer> idList = users.stream().map(User::getId).collect(Collectors.toList());
29 System.out.println(idList);
30
31 // 将id属性读取出来组成list 并去重
32 List<Integer> idList2 = users.stream().map(User::getId).distinct().collect(Collectors.toList());
33 System.out.println(idList2);
34
35 // 组装成map
36 Map<Integer, String> mp1 = users.stream().collect(Collectors.toMap(User::getId, User::getName));
37 System.out.println(mp1);
38
39 User user3_1 = new User(3, "3-1");
40 users.add(user3_1);
41
42 // 组装成map
43 // key 重复会报错
44 // Map<Integer, String> mp2 = users.stream().collect(Collectors.toMap(User::getId, User::getName));
45 // System.out.println(mp1);
46
47 // 组装成map 值取前面的
48 Map<Integer, String> mp3 = users.stream().collect(Collectors.toMap(User::getId, User::getName, (v1, v2) -> v1));
49 System.out.println(mp3);
50
51 // 组装成map 值取后面的
52 Map<Integer, String> mp2 = users.stream().collect(Collectors.toMap(User::getId, User::getName, (v1, v2) -> v2));
53 System.out.println(mp2);
54
55 // 组装成map 值取累加
56 Map<Integer, String> mp4 = users.stream().collect(Collectors.toMap(User::getId, User::getName, (v1, v2) -> v1+v2));
57 System.out.println(mp4);
58
59 // 根据id 组装list对象，对应id的对象成组
60 Map<Integer, List<User>> mp5 = users.stream().collect(Collectors.groupingBy(User::getId));
61 System.out.println(mp5);
62
63 // 根据id 组装list对象，对应id的对象成组,如果确认对象只有一个，
64 Map<Integer, User> mp8 = users.stream().collect(Collectors.toMap(User::getId, Function.identity(), (o1, o2) -> o2));
65 System.out.println(mp8);
66
67 // 根据id 组装list对象，对应id的对象的某个属性成组
68 Map<Integer,List<String>> mp6 = users.stream().collect(Collectors.groupingBy(User::getId, Collectors.mapping(User::getName, Collectors.toList())));
69 System.out.println(mp6);
70
71 Map<Integer, List<String>> mp7 = users.stream().collect(Collectors.toMap(User::getId,
72 p -> {
73 List<String> getNameList = new ArrayList<>();
74 getNameList.add(p.getName());
75 return getNameList;
76 },
77 (List<String> value1, List<String> value2) -> {
78 value1.addAll(value2);
79 return value1;
80 }
81 ));
82 System.out.println(mp7);
83
84 User user4 = new User(4, null);
85 users.add(user4);
86
87 // 属性为空值，也可以输出
88 Map<Integer, List<String>> mp8 = users.stream().collect(Collectors.toMap(User::getId,
89 p -> {
90 List<String> getNameList = new ArrayList<>();
91 getNameList.add(p.getName());
92 return getNameList;
93 },
94 (List<String> value1, List<String> value2) -> {
95 value1.addAll(value2);
96 return value1;
97 }
98 ));
99 System.out.println(mp8);
100
101 // 以对象为值
102 Map<Integer, User> mp9 = users.stream()
103 .filter(distinctByKey(User::getId))
104 .collect(Collectors.toMap(User::getId, User -> User));
105 System.out.println(mp9);
106 }
107
108 public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
109 Map<Object,Boolean> seen = new ConcurrentHashMap<>();
110 return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
111 }
112 }
113
114 class User{
115 private Integer id;
116 private String name;
117
118 public User() {
119 }
120
121 public User(Integer id, String name) {
122 this.id = id;
123 this.name = name;
124 }
125
126 public Integer getId() {
127 return id;
128 }
129
130 public void setId(Integer id) {
131 this.id = id;
132 }
133
134 public String getName() {
135 return name;
136 }
137
138 public void setName(String name) {
139 this.name = name;
140 }
141 }
