stream-groupby

/**
 * 背景
 * 当list映射成map时，会出现key重复异常？
 *使用分组
 *
 * */
public class Test {

    public static List<MerchPO> merchPOS;


    public static void main(String[] args) {
        MerchPO merchPO1 = new MerchPO("001", "888", "1", 0);
        MerchPO merchPO2 = new MerchPO("002", "888", "1", 100);
        MerchPO merchPO3 = new MerchPO("003", "888", "2", 200);
        MerchPO merchPO4 = new MerchPO("004", "999", "0", 300);

        //添加数据
        merchPOS = Stream.of(merchPO1, merchPO2, merchPO3, merchPO4).collect(Collectors.toList());

        Map<String, Map<String, List<MerchPO>>> stringMapMap = handleGroupByType5(merchPOS);
        System.out.println(stringMapMap);

    }

    //按照店铺类型分组-- 默认map={key, List<T>}
    public static Map<String, List<MerchPO>> handleGroupByType(List<MerchPO> list) {
        Map<String, List<MerchPO>> collect = list.stream().collect(Collectors.groupingBy(MerchPO::getType));
        return collect;
    }

    //按照店铺类型分组-- 改变映射结构 map={key, List<string>} Collectors.mapping()
    public static Map<String, List<String>> handleGroupByType2(List<MerchPO> list) {
        Map<String, List<String>> collect = list.stream().collect(Collectors.groupingBy(MerchPO::getType, Collectors.mapping(MerchPO::getMid, Collectors.toList())));
        return collect;
    }

    //按照店铺类型-统计个数 Collectors.counting()
    public static Map<String, Long> handleGroupByType3(List<MerchPO> list) {
        Map<String, Long> collect = list.stream().collect(Collectors.groupingBy(MerchPO::getType, Collectors.counting()));
        return collect;
    }


    //按照店铺类型-统计平均数距离 Collectors.counting()
    public static Map<String, Double> handleGroupByType4(List<MerchPO> list) {
        Map<String, Double> collect = list.stream().collect(Collectors.groupingBy(MerchPO::getType, Collectors.averagingDouble(MerchPO::getDistance)));
        return collect;
    }

    //嵌套分组--先分类，mno
    public static Map<String, Map<String, List<MerchPO>>> handleGroupByType5(List<MerchPO> list) {
        Map<String, Map<String, List<MerchPO>>> collect = list.stream().collect(Collectors.groupingBy(MerchPO::getType, Collectors.groupingBy(MerchPO::getMno)));
        return collect;

    }

    //分组后得到线程安全--ConcurrentMap
    public static Map<String, List<MerchPO>> handleGroupByType6(List<MerchPO> list) {
        ConcurrentMap<String, List<MerchPO>> collect = list.stream().collect(Collectors.groupingByConcurrent(MerchPO::getType));
        return collect;
    }

    //分组后，按照 类型排序,TreeMap 默认是升序
    public static Map<String, List<String>> handleGroupByType7(List<MerchPO> list) {
        TreeMap<String, List<String>> collect = list.stream().collect(Collectors.groupingBy(MerchPO::getType, TreeMap::new, Collectors.mapping(MerchPO::getMid, Collectors.toList())));
        Map<String, List<String>> stringListNavigableMap = collect.descendingMap();//反转一下就是，降序
        return stringListNavigableMap;

    }

}