源： java中的stream的Map收集器操作

   

package test9;

import java.util.Collections;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class CollectorsTest {
    public static void main(String[] args) {
        Map<String, Person> map = Stream
                .of(Person.valueOf("小明", '男', 18), Person.valueOf("小丽", '女', 16), Person.valueOf("小王", '男', 19))
                .collect(Collectors.toMap(Person::getName, Function.identity()));
        System.out.println("map:" + map);

        Map<String, Person> map2 = Stream
                .of(Person.valueOf("小明", '男', 18), Person.valueOf("小丽", '女', 16), Person.valueOf("小明", '男', 19))
                .collect(Collectors.toMap(Person::getName, Function.identity(), (x, y) -> y));
        System.out.println("如果有2个key重复了就用新的，‘y’,想用旧的代替就用'x',自己选择，map2:" + map2);

        /*
         * 需求：要求将已有的国家名为key, 人名为value(当然也可以是对象)集合。
         */
        Map<String, Set<String>> map3 = Stream
                .of(Person.valueOf("China", "小明"), Person.valueOf("China", "小丽"), Person.valueOf("us", "Jack"),
                        Person.valueOf("us", "Alice"))
                .collect(Collectors.toMap(Person::getCountry, p -> Collections.singleton(p.getName()), (x, y) -> {
                    Set<String> set = new HashSet<>(x);
                    set.addAll(y);
                    return set;
                }));
        System.out.println("map3:" + map3);

        // 将所有元素检查key不重复且最终包装成一个TreeMap对象
        Map<String, String> map4 = Stream.of(Person.valueOf("China", "小明"), Person.valueOf("Chinsa", "小丽"), Person.valueOf("us", "Jack"),
            Person.valueOf("uss", "Alice"))
            .collect(Collectors.toMap(
                Person::getCountry, 
                Person::getName, (x, y) -> 
                {
                    throw new IllegalStateException();
                },TreeMap::new));
        System.out.println(map4);

    }
}

class Person {
    private String country;

    private String name;

    private Character sex;

    private Integer age;

    public String getCountry() {
        return country;
    }

    public void setCountry(String country) {
        this.country = country;
    }

    public String getName() {
        return name;
    }

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

    public Character getSex() {
        return sex;
    }

    public void setSex(Character sex) {
        this.sex = sex;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "name:" + name + "sex:" + sex + "age" + age;
    }

    public static Person valueOf(String country, String name) {
        Person person = new Person();
        person.country = country;
        person.name = name;
        return person;
    }

    public static Person valueOf(String name, Character sex, Integer age) {
        Person person = new Person();
        person.name = name;
        person.sex = sex;
        person.age = age;
        return person;
    }
}

打印结果：

map:{小明=name:小明sex:男age18, 小丽=name:小丽sex:女age16, 小王=name:小王sex:男age19}

如果有2个key重复了就用新的，‘y’,想用旧的代替就用'x',自己选择，map2:{小明=name:小明sex:男age19, 小丽=name:小丽sex:女age16}

map3:{China=[小明, 小丽], us=[Alice, Jack]}

{China=小明, Chinsa=小丽, us=Jack, uss=Alice}