java-comparator

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要使自己的类拥有排序功能，就要实现comparator接口,重写compare方法。

原题链接：Java Comparator

Comparators are used to compare two objects. In this challenge, you’ll create a comparator and use it to sort an array.

The Player class is provided for you in your editor. It has fields: a String and a integer.

Given an array of Player objects, write a comparator that sorts them in order of decreasing score; if or more players have the same score, sort those players alphabetically by name. To do this, you must create a Checker class that implements the Comparator interface, then write an int compare(Player a, Player b) method implementing the Comparator.compare(T o1, T o2) method.

Sample Input

5
amy 100
david 100
heraldo 50
aakansha 75
aleksa 150

Sample Output

aleksa 150
amy 100
david 100
aakansha 75
heraldo 50

代码如下:

import java.util.*;

class Checker implements Comparator<Player> {
    @Override
    public int compare(Player p1, Player p2) {
        if (p2.score == p1.score) {
            return p1.name.compareTo(p2.name);
        } else {
            return p1.score > p2.score ? -1 : 1;    
        }
    }
}

class Player {
    String name;
    int score;
    
    Player(String name, int score) {
        this.name = name;
        this.score = score;
    }
}

class Solution {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        Player[] player = new Player[n];
        Checker checker = new Checker();
        for(int i = 0; i < n; i++) {
            player[i] = new Player(scan.next(), scan.nextInt());
        }
        scan.close();  
        Arrays.sort(player, checker);
        for(int i = 0; i < player.length; i++) {
            System.out.printf("%s %s\n", player[i].name, player[i].score);
        }
    }
}

知识点补充：

1. String类有个compareTo()函数，前者比后者小时返回负数。
2. public int compare(Player p1, Player p2)返回负数时，不换位置。
3. 注意用法：Arrays.sort(player, checker);

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原题链接：Java Sort

You are given a list of student information: ID, FirstName, and CGPA. Your task is to rearrange them according to their CGPA in decreasing order. If two student have the same CGPA, then arrange them according to their first name in alphabetical order. If those two students also have the same first name, then order them according to their ID. No two students have the same ID.

Sample Input

5
33 Rumpa 3.68
85 Ashis 3.85
56 Samiha 3.75
19 Samara 3.75
22 Fahim 3.76

Sample Output

Ashis
Fahim
Samara
Samiha
Rumpa

代码如下:

import java.util.*;

class Student{
   private int id;
   private String fname;
   private double cgpa;
   public Student(int id, String fname, double cgpa) {
      super();
      this.id = id;
      this.fname = fname;
      this.cgpa = cgpa;
   }
   public int getId() {
      return id;
   }
   public String getFname() {
      return fname;
   }
   public double getCgpa() {
      return cgpa;
   }
}

class StudentComparator implements Comparator<Student>{
	@Override
	public int compare(Student s1, Student s2) {
		double cgpa1 = s1.getCgpa();
		double cgpa2 = s2.getCgpa();
		if(Math.abs(cgpa1 - cgpa2)<0.00000001){
			int fnameCom = s1.getFname().compareTo(s2.getFname());
			if(fnameCom==0)	return s1.getId()-s2.getId(); 
			else return fnameCom;
		}
		else return (cgpa1<cgpa2)?1:-1;
	}
}

public class Solution
{
	public static void main(String[] args){
		Scanner in = new Scanner(System.in);
		int testCases = Integer.parseInt(in.nextLine());
		List<Student> studentList = new ArrayList<Student>();
		while(testCases>0){
			int id = in.nextInt();
			String fname = in.next();
			double cgpa = in.nextDouble();
			Student st = new Student(id, fname, cgpa);
			studentList.add(st);
			testCases--;
		}
		Collections.sort(studentList, new StudentComparator());
		for(Student st: studentList){
			System.out.println(st.getFname());
		}
	}
}

知识点补充：

这题用了容器ArrayList，所以重写好Comparator接口后，排序时要用Collections.sort(studentList, new StudentComparator());