利用google工具类实现对list集合分组

 @org.junit.Test
    public void testTest6() {
        Person p1 = new Person("a",10,"男","china");
        Person p2 = new Person("b",11,"女","japan");
        Person p3 = new Person("c",12,"男","japan");
        Person p4 = new Person("d",14,"男","USA");
        Person p5 = new Person("e",14,"女","china");
        List<Person> persons = new ArrayList<>();
        persons.add(p1);
        persons.add(p2);
        persons.add(p3);
        persons.add(p4);
        persons.add(p5);
        ImmutableSet digits = ImmutableSet.of(persons.toArray());
        Function<Person, String> sex = new Function<Person, String>() {
            @Override
            public String apply(Person person) {
                return person.getSex();
            }
        };
        ImmutableListMultimap<String, Person> sexList = Multimaps.index(digits, sex);

        ImmutableListMultimap ageList = Multimaps.index(digits, new Function<Person, Integer>() {
            @Override
            public Integer apply(Person person) {
                return person.getAge();
            }
        });

        ImmutableListMultimap countryList = Multimaps.index(digits, new Function<Person, String>() {
            @Override
            public String apply(Person person) {
                return person.getContry();
            }
        });
        System.out.println("按性别分组 = " + sexList);
        System.out.println("按年龄分组 = " + ageList);
        System.out.println("按国家分组 = " + countryList);

        /** 结果
         * 按性别分组 = {男=[Person@44d03877, Person@215750e4, Person@6b7fb9d5], 女=[Person@422b2fec, Person@e818616]}
         * 按年龄分组 = {10=[Person@44d03877], 11=[Person@422b2fec], 12=[Person@215750e4], 14=[Person@6b7fb9d5, Person@e818616]}
         * 按国家分组 = {china=[Person@44d03877, Person@e818616], japan=[Person@422b2fec, Person@215750e4], USA=[Person@6b7fb9d5]}
         */
        //求性别为男的集合
        System.out.println("sexList = " + sexList.get("男"));
        //求年龄为14岁的集合
        System.out.println("ageList = " + ageList.get(14));
        //求国籍日本的集合
        System.out.println("countryList = " + countryList.get("japan"));
        /**
         * 结果
         * sexList = [Person@e818616, Person@789caeb2, Person@769165fa]
         * ageList = [Person@769165fa, Person@43be3ce6]
         * countryList = [Person@598a15ca, Person@789caeb2]
         * 以上结果就是根据返回的map得到的集合数据。
         * 可以把结果用List<Person> 保存。
         * List<Person> personMan = sexList.get("男");
         */
    }