Java8之stream流的分组排序

关于Java8的stream流，这里不讲groupBy分组，也不讲sort排序，这些都是很基础的用法，可以自行百度。

这里说一种业务场景，对于分组后的map，根据value对key-value进行排序。举个例子，人（姓名，地址，创建时间）的集合，要求按地址将他们分组，同时要求越晚被创建的人，所在的分组越靠前。

直接上People类：

import lombok.AllArgsConstructor;
import lombok.Data;

@Data
@AllArgsConstructor
public class People {

    private String name;

    private String address;

    private Long createTime;

}

然后是分组排序代码：

	public static void main(String[] args) {
		long time = 1L;
		List<People> list = new ArrayList<>();
		list.add(new People("曹丕", "魏", time++));
		list.add(new People("关羽", "蜀", time++));
		list.add(new People("刘备", "蜀", time++));
		list.add(new People("小乔", "吴", time++));
		list.add(new People("周瑜", "吴", time++));
		list.add(new People("曹操", "魏", time++));

		Map<String, List<People>> collect = list.stream()
				.filter(d -> d != null && d.getAddress() != null) // 这里是为了下面分组的key不为null，要是key为null会报错的
				.sorted(Comparator.comparingLong(People::getCreateTime).reversed())
				.collect(Collectors.groupingBy(People::getAddress, LinkedHashMap::new, Collectors.toList()));

		for (Map.Entry<String, List<People>> entry : collect.entrySet()) {
			System.out.println(entry.getKey() + "\t" + entry.getValue());
		}
	}

最后是运行结果：

魏	[People(name=曹操, address=魏, createTime=6), People(name=曹丕, address=魏, createTime=1)]
吴	[People(name=周瑜, address=吴, createTime=5), People(name=小乔, address=吴, createTime=4)]
蜀	[People(name=刘备, address=蜀, createTime=3), People(name=关羽, address=蜀, createTime=2)]

那么如果按照题目要求，这个时候，我再加个“蜀国”人，那么这个人所在分组就应该放到第一的位置

		long time = 1L;
		List<People> list = new ArrayList<>();
		list.add(new People("曹丕", "魏", time++));
		list.add(new People("关羽", "蜀", time++));
		list.add(new People("刘备", "蜀", time++));
		list.add(new People("小乔", "吴", time++));
		list.add(new People("周瑜", "吴", time++));
		list.add(new People("曹操", "魏", time++));
		list.add(new People("张飞", "蜀", time++));

运行后结果如下，发现最后添加的“张飞”所在的“蜀”分类，已经放到了第一的位置：

蜀	[People(name=张飞, address=蜀, createTime=7), People(name=刘备, address=蜀, createTime=3), People(name=关羽, address=蜀, createTime=2)]
魏	[People(name=曹操, address=魏, createTime=6), People(name=曹丕, address=魏, createTime=1)]
吴	[People(name=周瑜, address=吴, createTime=5), People(name=小乔, address=吴, createTime=4)]