Created
January 24, 2018 03:38
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Converting Collection to Map with JDK 8 Collectors
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| package com.example; | |
| import static java.lang.System.out; | |
| import java.util.*; | |
| import java.util.function.BinaryOperator; | |
| import java.util.stream.Collectors; | |
| public class Main { | |
| private static final User[] users; | |
| static { | |
| users = new User[]{ | |
| new User("u-1", "rich", 30), | |
| new User("u-2", "tom", 20), | |
| new User("u-1", "rich lee", 30), | |
| new User("u-3", "Jackson", 45) | |
| }; | |
| } | |
| private static Map<String, User> convertArrayToMap(final User[] users) { | |
| return Arrays.stream(users).collect( | |
| Collectors.toMap(User::getId, user -> user)); | |
| } | |
| private static Map<String, User> convertListToMap(final List<User> users) { | |
| return users.stream().collect( | |
| Collectors.toMap(User::getId, user -> user)); | |
| } | |
| /** | |
| * Use initial value when the mapped keys contains duplicates. | |
| * | |
| * @param users | |
| * @return | |
| */ | |
| private static Map<String, User> convertListToMapAndMerge(final List<User> users) { | |
| return users.stream().collect( | |
| Collectors.toMap(User::getId, user -> user, usingInitialValue())); | |
| } | |
| private static Map<String, User> convertSetToMap(final Set<User> userSet) { | |
| return userSet.stream().collect( | |
| Collectors.toMap(User::getId, user -> user)); | |
| } | |
| private static <T> BinaryOperator<T> usingInitialValue() { | |
| return (a,b) -> a; | |
| } | |
| private static <T> BinaryOperator<T> usingLastValue() { | |
| return (a,b) -> b; | |
| } | |
| public static void main(String[] args) { | |
| final List<User> usersList = Arrays.asList(users); | |
| /* | |
| If the mapped keys contains duplicates (according to Object.equals(Object)), an IllegalStateException is | |
| thrown when the collection operation is performed. | |
| If the mapped keys may have duplicates, use toMap(Function keyMapper, Function valueMapper, BinaryOperator mergeFunction) instead. | |
| */ | |
| // out.println("LIST -> MAP:\n" + convertListToMap(usersList)); | |
| out.println("LIST -> MAP:\n" + convertListToMapAndMerge(usersList)); | |
| } | |
| } |
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| package com.example; | |
| public class User { | |
| final String id; | |
| final String name; | |
| final int age; | |
| public User(String id, String name, int age) { | |
| this.id = id; | |
| this.name = name; | |
| this.age = age; | |
| } | |
| public String getId() { | |
| return id; | |
| } | |
| public String getName() { | |
| return name; | |
| } | |
| public int getAge() { | |
| return age; | |
| } | |
| @Override | |
| public String toString() { | |
| return "User{" + | |
| "id='" + id + '\'' + | |
| ", name='" + name + '\'' + | |
| ", age=" + age + | |
| '}'; | |
| } | |
| } |
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