《Java8实战》05 - Stream流的Collectors实现: 将POJO实体列表转为<Id, POJO>的Map 或者 <Id, 某个属性> 一对一的KV键值对

笔记简记

1. Collectors.groupingBy 简单使用
2. Collectors.toMap 简单使用

数据准备 POJO测试类

@Data
class InnerClass {
    private Long id;
    private String name;
    private LocalDateTime birthDay;
    public InnerClass() {
    }
    InnerClass(Long id, String name, LocalDateTime desc) {
        this.id = id;
        this.name = name;
        this.birthDay = desc;
    }
}

实现<Id, POJO>的一对一Map代码及实现

 InnerClass clazz1 = new InnerClass(1L, "name1", LocalDateTime.now());
        InnerClass clazz2 = new InnerClass(2L, "name2", LocalDateTime.now().minusYears(1));
        InnerClass clazz3 = new InnerClass(3L, "name3", LocalDateTime.now().minusYears(2));

        ArrayList<InnerClass> allList = new ArrayList<>();
        allList.add(clazz1);
        allList.add(clazz2);
        allList.add(clazz3);

        // (Collectors.groupingBy() 的最简单的默认接口实现
        //     public static <T, K> Collector<T, ?, Map<K, List<T>>>
        //    groupingBy(Function<? super T, ? extends K> classifier) {
        //        return groupingBy(classifier, toList());
        //    }

        Map<Long, List<InnerClass>> collect = allList.stream().collect(Collectors.groupingBy(InnerClass::getId));
        collect.forEach((k, v) -> {
            System.out.println("<Id, 对实体>  " + k + "  " + v);
        });
        // 生成<Id, 对实体>的Map 打印如下::::::::::::::::::::::::
        //1  [InnerClass{id=1, name='name1', birthDay=2020-06-12T16:17:54.150}]
        //2  [InnerClass{id=2, name='name2', birthDay=2019-06-12T16:17:54.151}]
        //3  [InnerClass{id=3, name='name3', birthDay=2018-06-12T16:17:54.151}]

<Id, 某个属性> 一对一的KV键值对

       /**
         * 创建<Id, name> 一对一的KV键值对list
         */

        Map<Long, String> longStringMap = allList.stream().collect(Collectors.toMap(InnerClass::getId, InnerClass::getName));
        longStringMap.forEach((k, v) -> {
            System.out.println("<Id, name>\t" + k + "  " + v);
        });
        //<Id, name>	1  name1
        //<Id, name>	2  name2
        //<Id, name>	3  name3

创建<Id, List > 一对多的KV键值对list

        /**
         * 创建<Id, List<name> > 一对多的KV键值对list
         */
        InnerClass clazz22 = new InnerClass(2L, "name222", LocalDateTime.now().minusYears(1));
        InnerClass clazz33 = new InnerClass(3L, "name333", LocalDateTime.now().minusYears(2));
        allList.add(clazz22);
        allList.add(clazz33);

        Map<Long, List<InnerClass>> collect1 = allList.stream().collect(Collectors.groupingBy(InnerClass::getId));
        collect1.forEach((k, v) -> {
            System.out.println("<Id, List<实体> >  " + k + "  " + v);
        });
         // 打印如下::::::::::::::::::::::::
        //<Id, List<实体> >  1  [InnerClass{id=1, name='name1', birthDay=2020-06-12T16:48:29.381}]
        //<Id, List<实体> >  2  [InnerClass{id=2, name='name2', birthDay=2019-06-12T16:48:29.382}, InnerClass{id=2, name='name222', birthDay=2019-06-12T16:48:29.404}]
        //<Id, List<实体> >  3  [InnerClass{id=3, name='name3', birthDay=2018-06-12T16:48:29.382}, InnerClass{id=3, name='name333', birthDay=2018-06-12T16:48:29.404}]