Operation of Java8 to convert an Listless thanTgreater than to Mapless thanString Tgreater than

Convert List to Map < String, T >

public class AnswerApp {
 public static void main(String[] args) throws Exception {
  List<String> names = Lists.newArrayList("Answer", "AnswerAIL", "AI");
  Map<String, Integer> map = names.stream().collect(Collectors.toMap(v -> v, v -> 1));
  System.out.println(map);
 }
}

Program running output

{Answer=1, AnswerAIL=1, AI=1}

Convert List to Map < K, V >

public static void main(String[] args) throws Exception {
 List<User> users = new ArrayList<>();
 for (int i = 0; i < 3; i++) {
  users.add(new User("answer" + i, new Random().nextInt(100)));
 }
 System.out.println(JSON.toJSONString(users)); 
 System.out.println(); 
 Map<String, Integer> map = users.stream().collect(Collectors.toMap(User::getName, User::getAge));
 System.out.println(map);
}

Program running output

[{"age":78,"name":"answer0"},{"age":89,"name":"answer1"},{"age":72,"name":"answer2"}]
{answer2=72, answer1=89, answer0=78}

Convert List to Map < String, T >

Implementation Mode 1

public class AnswerApp {
 public static void main(String[] args) throws Exception {
  List<User> users = new ArrayList<>();
  for (int i = 0; i < 3; i++) {
   //  Change to this code ,  Turn map Errors will be reported when  Duplicate key User
 // users.add(new User("answer", new Random().nextInt(100)));
   users.add(new User("answer" + i, new Random().nextInt(100)));
  }
  System.out.println(JSON.toJSONString(users));
  System.out.println();
  Map<String, User> map = users.stream().collect(Collectors.toMap(User::getName, Function.identity()));
  System.out.println(JSON.toJSONString(map));
 }
}

In this way, if key of map (such as the value of User:: getName in the above example) is duplicated, java. lang. IllegalStateException: ES40key User will be thrown incorrectly

Program running output

[{"age":22,"name":"answer0"},{"age":79,"name":"answer1"},{"age":81,"name":"answer2"}]
{"answer2":{"age":81,"name":"answer2"},"answer1":{"age":79,"name":"answer1"},"answer0":{"age":22,"name":"answer0"}}

Implementation 2

public class AnswerApp {
 public static void main(String[] args) throws Exception {
  List<User> users = new ArrayList<>();
  for (int i = 0; i < 3; i++) {
   users.add(new User("answer", new Random().nextInt(100)));
  }
  System.out.println(JSON.toJSONString(users));
  System.out.println();
 
 //  If  key  Repetition ,  Then according to   Conflict method   · (key1, key2) -> key2 ·   Judge .  Explanation : key1 key2  When in conflict   Take  key2 
  Map<String, User> map = users.stream().collect(Collectors.toMap(User::getName, Function.identity(), (key1, key2) -> key2));
  System.out.println(JSON.toJSONString(map));
 }
} 

Program running output

[{"age":24,"name":"answer"},{"age":89,"name":"answer"},{"age":68,"name":"answer"}]
{"answer":{"age":68,"name":"answer"}}

If changed to (key1, key2)- > key1 outputs {"answer": {"age": 24, "name": "answer"}}

User entity

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
 private Long id;
 private String name;
 private Integer age;
 public User(String name) {
  this.name = name;
 }
 public User(String name, Integer age) {
  this.name = name;
  this.age = age;
 }
}

Supplement: Lambda expression is used in java8 to transfer two fields of entity class in list to Map

Code:

List<Entity> list = new ArrayList<>();
Map<Integer, String> map = list.stream().collect(Collectors.toMap(Entity::getId, Entity::getType));

Common lambda expressions:

{Answer=1, AnswerAIL=1, AI=1}

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