java - 获取对象的 ArrayList 的最小/最大浮点值

Question

我有一个包含浮点属性的 Person ArrayList。我想做的是获取我的 Arraylist 的最小值和最大值以显示它们。

public class Person implements Serializable {

    private float myvalue;
    private Date date;
    //getters setters...

我尝试使用 Collections.min(mylist) 和 Collections.max(mylist) 但似乎我必须覆盖比较器。

然后我的第二个问题是我想在 date 属性中找到 mylist 中具有相同月份(同一年)的每个元素，但我真的不知道我该怎么做这样做。

希望有人能帮助我!

Answer 1

假设您有一个人员列表...

    Collection<Person> people = new ArrayList<>();

这就是你如何获得最大和最小值(value)的人

    Person maxValuePerson = people.parallelStream()
            .max(Comparator.comparing(p -> ((Person) p).getMyValue()))
            .get();
    Person minValuePerson = people.parallelStream()
            .min(Comparator.comparing(p -> ((Person) p).getMyValue()))
            .get();

然后您可以使用 Map<People> 按月对人员进行分组和一个 Calendar实例如下:

    HashMap<Integer,ArrayList<Person>> monthMap = new HashMap<>();

    Calendar cal = Calendar.getInstance(); //expensive operation... use sparingly

    for (Person p : people){
        cal.setTime(p.getDate()); //Sets this Calendar's time with the person's Date.
        int month = cal.get(Calendar.MONTH); //gets int representing the month
        ArrayList<Person> monthList = monthMap.get(month); 

        //initialize list if it's null (not already initialized)
        if(monthList == null) {
            monthList = new ArrayList<>(); 
        }

        monthList.add(p); //add the person to the list

        // put this month's people list into the map only if it wasn't there to begin with
        monthMap.putIfAbsent(month, monthList); 
    }

综上所述，这是一个您可以测试的完整工作示例:

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Collection;
import java.util.Comparator;
import java.util.Date;
import java.util.HashMap;
import java.util.Random;

public class MinMaxTest {

    public static void main(String[] args) {

        Random rand = new Random();


        //Assuming an array list of people...
        Collection<Person> people = new ArrayList<>();


        for (int i = 0; i < 50; i++){
            Person p = new Person();
            p.setMyvalue(rand.nextFloat());
            p.setDate(new Date(rand.nextLong()));
            people.add(p);
        }

        //This is how you get the max and min value people
        Person maxValuePerson = people.parallelStream()
                .max(Comparator.comparing(p -> ((Person) p).getMyValue()))
                .get();
        Person minValuePerson = people.parallelStream()
                .min(Comparator.comparing(p -> ((Person) p).getMyValue()))
                .get();

        //to group the people by month do the following:
        HashMap<Integer,ArrayList<Person>> monthMap = new HashMap<>();

        Calendar cal = Calendar.getInstance();

        for (Person p : people){
            cal.setTime(p.getDate());
            int month = cal.get(Calendar.MONTH);
            ArrayList<Person> monthList = monthMap.get(month);
            if(monthList == null)
                monthList = new ArrayList<>();
            monthList.add(p);
            monthMap.putIfAbsent(month, monthList);
        }

        for(Integer i : monthMap.keySet()){
            System.out.println("Month: "+ i);
            for(Person p : monthMap.get(i)){
                System.out.println(p);
            }
        }

    }

    static class Person implements Serializable {
        private float myvalue;
        private Date date;

        public Date getDate() {
            return date;
        }
        public void setDate(Date date) {
            this.date = date;
        }
        public float getMyValue() {
            return myvalue;
        }
        public void setMyvalue(float myvalue) {
            this.myvalue = myvalue;
        }
    }

}