Java8 Stream()关于在所有用户的所有上传记录中，找出每个用户最新上传记录

原创文章：转载请标明出处 https://www.cnblogs.com/yunqing/p/9504196.html

首先分析相当于如下，在所有的猫中，每个名字的猫都保留年龄最小的一个

import java.util.ArrayList;
import java.util.Comparator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

public class test {

	public static void main(String[] args) {
		List<Cat> list = new ArrayList<>();
		list.add(new Cat("小花",1,1));
		list.add(new Cat("小花",2,1));
		list.add(new Cat("小2",3,1));
		list.add(new Cat("小2",4,1));
		list.add(new Cat("小3",5,1));
		list.add(new Cat("小3",7,1));
		list.add(new Cat("小花",3,1));
		list.add(new Cat("小1",1,1));
		list.add(new Cat("小1",2,1));
		list.add(new Cat("小1",3,1));
		list.add(new Cat("小3",3,1));
		list.add(new Cat("小3",12,1));

		List<Cat> result = new LinkedList<>();

		Map<String, List<Cat>> res = list.stream().collect(Collectors.groupingBy(Cat::getName));
		Set<String> keys = res.keySet();

		for (String str : keys) {
			List<Cat> groupList = list.stream().filter(e->e.getName().equals(str)).collect(Collectors.toList());
			//以下两行都是找出组内年龄最小的猫
			//Cat catMinAge0 = groupList.stream().reduce((one, two) -> one.getAge() < two.getAge() ? one : two).orElse(null);
			Cat catMinAge1 = groupList.stream().min(Comparator.comparingInt(o -> o.getAge())).orElse(null);
			result.add(catMinAge1);
		}

		System.out.println(result);

	}

}

class Cat{
	private String name;
	private int age;
	private int gender;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	public int getGender() {
		return gender;
	}
	public void setGender(int gender) {
		this.gender = gender;
	}
	public Cat() {}
	public Cat(String name, int age, int gender) {
		super();
		this.name = name;
		this.age = age;
		this.gender = gender;
	}
	@Override
	public String toString() {
		return "Cat [name=" + name + ", age=" + age + ", gender=" + gender + "]";
	}

}

　　测试完毕，拿到项目中使用如下：

	@Override
	@Transactional
	public List<DrivingRecord> findByCityCodeAndRecordTimeStartingWith(String cityCode, String recondTime) {
		//查询此区域今天所有用户的所有上报记录
		List<DrivingRecord> list = drivingRecondDao.findByCityCodeAndRecordTimeStartingWith(cityCode, recondTime);
		List<DrivingRecord> resultList = new ArrayList<>();
		//先根据用户分组，把同一个用户的上报记录放到同一个组
		Map<User, List<DrivingRecord>> map = list.stream()
							 .collect(Collectors.groupingBy(DrivingRecord::getUser));
		//取出所有用户key值
		Set<User> keys = map.keySet();
		//遍历所有用户
		for (User user : keys) {
			//找出与当前用户的分组
			List<DrivingRecord> groupList = list.stream()
							    .filter(u->u.getUser().equals(user))
							    .collect(Collectors.toList());
			//比较时间，取当前用户的最新上报的记录
			DrivingRecord dr = groupList.stream()
						    .reduce((one,two)->one.getRecordTime().isBefore(two.getRecordTime())?two:one)
						    .orElse(null);
			//把所有用户的最新上报信息填到resultList
			resultList.add(dr);
		}

		return resultList;
	}