關於Java8的stream流，這裡不講groupBy分組，也不講sort排序，這些都是很基礎的用法，可以自行百度。

這裡說一種業務場景，對於一些人（姓名，地址，建立時間），要求按地址將他們分組，同時要求越晚被建立的人，所在的分組越靠前。

直接上People類：

import lombok.AllArgsConstructor;
import lombok.Data;

@Data
@AllArgsConstructor
public class People {

    private String name;

    private String address;

    private Long createTime;

}
 

然後是分組排序程式碼：

	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("關羽", "蜀", time++));
		list.add(new People("劉備", "蜀", time++));
		list.add(new People("小喬", "吳", time++));
		list.add(new People("周瑜", "吳", time++));
		list.add(new People("曹操", "魏", time++));

		Map<String, List<People>> collect = list.stream()
				.filter(d -> d != null && d.getAddress() != null)
				.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());
		}
	}
 

最後是執行結果：

魏	[People(name=曹操, address=魏, createTime=6), People(name=曹丕, address=魏, createTime=1)]
吳	[People(name=周瑜, address=吳, createTime=5), People(name=小喬, address=吳, createTime=4)]
蜀	[People(name=劉備, address=蜀, createTime=3), People(name=關羽, address=蜀, createTime=2)]

那麼如果按照題目要求，這個時候，我再加個“蜀國”人，那麼這個人所在分組就應該放到第一的位置

		long time = 1L;
		List<People> list = new ArrayList<>();
		list.add(new People("曹丕", "魏", time++));
		list.add(new People("關羽", "蜀", time++));
		list.add(new People("劉備", "蜀", time++));
		list.add(new People("小喬", "吳", time++));
		list.add(new People("周瑜", "吳", time++));
		list.add(new People("曹操", "魏", time++));
		list.add(new People("張飛", "蜀", time++));

執行後結果如下，發現最後新增的“張飛”所在的“蜀”分類，已經放到了第一的位置：

蜀	[People(name=張飛, address=蜀, createTime=7), People(name=劉備, address=蜀, createTime=3), People(name=關羽, address=蜀, createTime=2)]
魏	[People(name=曹操, address=魏, createTime=6), People(name=曹丕, address=魏, createTime=1)]
吳	[People(name=周瑜, address=吳, createTime=5), People(name=小喬, address=吳, createTime=4)]