Stream流的collect转化到Map映射

package com.baifan.collecting;

import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
import java.util.function.Function;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * @author: baifan
 * @date: 2021/2/5
 */
public class CollectingIntoMaps {

    public static class Person {
        private int id;
        private String name;

        public Person(int id, String name) {
            this.id = id;
            this.name = name;
        }

        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        @Override
        public String toString() {
            return "Person{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    '}';
        }
    }

    public static Stream<Person> people() {
        return Stream.of(new Person(1001, "张三"), new Person(1002, "李四"), new Person(1003, "王五"));
    }

    public static void main(String[] args) {
        Map<Integer, String> idToName = people().collect(Collectors.toMap(Person::getId, Person::getName));
        System.out.println("idToName:" + idToName);

        Map<Integer, Person> idToPerson = people().collect(Collectors.toMap(Person::getId, Function.identity()));
        System.out.println("idToPerson:" + idToPerson.getClass().getName() + idToPerson);

        idToPerson = people().collect(Collectors.toMap(Person::getId, Function.identity(), (existingValue, newValue) -> {
            throw new IllegalStateException();
        }, TreeMap::new));
        System.out.println("idToPerson:" + idToPerson.getClass().getName() + idToPerson);

        Stream<Locale> locales = Stream.of(Locale.getAvailableLocales());
        Map<String, String> languageNames = locales.collect(Collectors.toMap(
                Locale::getDisplayName,
                locale -> locale.getDisplayLanguage(locale),
                (existingValue, newValue) -> existingValue
        ));
        System.out.println("languageNames:" + languageNames);


        locales = Stream.of(Locale.getAvailableLocales());
        Map<String, Set<String>> countryLanguageSets = locales.collect(
                Collectors.toMap(Locale::getDisplayCountry, locale -> Collections.singleton(locale.getDisplayLanguage()),
                        (a, b) -> {// union of a and b
                            Set<String> union = new HashSet<>(a);
                            union.addAll(b);
                            return union;
                        }));
        System.out.println("countryLanguageSets:" + countryLanguageSets);
    }
}