实体类

public class User {

    private Integer id;

    private String username;

    private Integer age;

}

1.Guava方法

/**
     * uniqueIndex(唯一索引): 通过指定key值创建Map
     *
     * 但是该方法存在两个问题:
     * 1. 数据来源的集合指定为key值的属性的值不能出现重复的情况
     * 2. key对应的value是唯一的,不是集合,不能器到分类的作用
     *
     */
    public static void guavaMethod() {
        List<User> users = Lists.newArrayList();
        users.add(new User(1, "Joe", 20));
        users.add(new User(2, "Tony", 23));
        users.add(new User(3, "Mary", 18));
        ImmutableMap<Integer, User> integerUserImmutableMap = FluentIterable.from(users).uniqueIndex(c -> c.getAge());
        ImmutableMap<Integer, User> integerUserImmutableMap1 = Maps.uniqueIndex(users, c -> c.getAge());
    }

/**
     * Multimaps.index():
     *
     * 创建的Map的key是可以重复的
     */
    public static void Multimaps() {
        List<User> users = Lists.newArrayList();
        users.add(new User(1, "Joe", 20));
        users.add(new User(2, "Tony", 23));
        users.add(new User(3, "Mary", 18));
        users.add(new User(4, "Henny", 18));
        ImmutableListMultimap<Integer, User> index = Multimaps.index(users, c -> c.getAge());
    }

2.Java8

 /**
     * 通过Java8来实现List转Map
     *
     * toMap可以指定key,value的值
     *
     */
    public static void java8Method() {
        List<User> users = Lists.newArrayList();
        users.add(new User(1, "Joe", 20));
        users.add(new User(2, "Tony", 23));
        users.add(new User(3, "Mary", 18));
        Map<Integer, User> collect = users.stream().collect(Collectors.toMap(c -> c.getAge(), p -> p));
    }

/**
     * partitioningBy(分割) 和 的作用相识
     * 不同的是他的key值为true/false
     *
     * @return
     */
    public static void groupingBy() {
        List<User> users = Lists.newArrayList();
        users.add(new User(1, "Joe", 20));
        users.add(new User(2, "Tony", 23));
        users.add(new User(3, "Mary", 18));
        users.add(new User(4, "Henny", 18));
        HashMap<Integer, List<User>> collect =
                users.stream().collect(Collectors.groupingBy(c -> c.getAge(), HashMap::new, Collectors.toList()));
        
        Map<Integer, List<Integer>> collect1 = users.stream()
                .collect(Collectors.groupingBy(c -> c.getAge(), Collectors.mapping(p -> p.getId(), Collectors.toList())));
    }