如何使用比较器对数组中的某些特定元素进行排序

Question

I am trying to sort an array in a specific order. if number % 10 is equal 0 then it will come first and for the rest, if number mod 10 is greater it will come first.

Collections.sort(arr,new Comparator<Integer>(){
   public int compare(Integer e1, Integer e2) {
       if(e1 %10 == 0 && e2 %10 != 0  || (e2 %10 == 0 && e1 %10 != 0)  
       || e1 % 10 ==0 || e2 % 10 ==0){
           return -1;
       }else{
           if(e1 % 10 < e2 % 10){
               return 1 ;
           }else{
               return -1 ;
           }
       }
   }});

input : [120, 20, 35, 7, 29]

output: [29, 7, 35, 20, 120]

expected: [120, 20, 29, 7 , 35]

Answer 1

import java.util.*; 
import java.lang.*; 
import java.io.*; 
public class Main
{
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>();
        list.add(120);
        list.add(20);
        list.add(35);
        list.add(7);
        list.add(29);

        Collections.sort(list,new Comparator<Integer>(){
           public int compare(Integer e1, Integer e2) {
               if(e1 %10 == 0  || e2%10 == 0){
                   return 1;
               }
               else {
                   if(e1 % 10 > e2 % 10){
                       return -1 ;
                   }else{
                       return 0 ;
                   }
               }
           }});

           Iterator<Integer> myListIterator = list.iterator(); 
           while(myListIterator.hasNext()) {
               System.out.println(myListIterator.next());
           }
    }
}

Answer 2

A comparator which return 0 indicates that the values are equal and nothing need to be switched. In your case, we have the say in the compare method, that the first if should return 0. This will give you the wanted result.

Collections.sort(arr, new Comparator<Integer>() {
    public int compare(Integer e1, Integer e2) {
        if (e1 % 10 == 0 && e2 % 10 != 0 || (e2 % 10 == 0 && e1 % 10 != 0) || e1 % 10 == 0 || e2 % 10 == 0) {
            return 0;
        } else {
            if (e1 % 10 < e2 % 10) {
                return 1;
            } else {
                return -1;
            }
        }
    }
});

Answer 3

The simplest way to do this with an explicit comparator is:

public int compareTo(Integer a, Integer b) {
  int cmp = Boolean.compare(a % 10 != 0, b % 10 != 0);
  if (cmp != 0) {
    return cmp;
  }
  return a.compareTo(b);
}

Using Boolean.compare(a % 10 != 0, b % 10 != 0) finds if the last digit is zero or not: if a does not end with zero but b does, a positive value is returned, so a will be sorted after b .

If the either both end with a zero, or neither ends with a zero, the numbers are compared naturally.