Using Collections.sort and Comparator in Java

Sorting a collection in Java is easy, just use Collections.sort(Collection) to sort your values. For example:

package de.vogella.algorithms.sort.standardjava;
 
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
 
public class Simple {
    public static void main(String[] args) {
        List list = new ArrayList();
        list.add(5);
        list.add(4);
        list.add(3);
        list.add(7);
        list.add(2);
        list.add(1);
        Collections.sort(list);
        for (Integer integer : list) {
            System.out.println(integer);
        }
    }
}

This is possible because Integer implements the Comparable interface. This interface defines the method compare which performs pairwise comparison of the elements and returns -1 if the element is smaller then the compared element, 0 if it is equal and 1 if it is larger.

package de.vogella.algorithms.sort.standardjava;
 
import java.util.Comparator;
 
public class MyIntComparable implements Comparator<Integer>{
 
    @Override
    public int compare(Integer o1, Integer o2) {
        return (o1>o2 ? -1 : (o1==o2 ? 0 : 1));
    }
}

But what if what to sort differently, e.g. for example in different order. Well, you could just use Collection.reverse(). Or you define your own class with implements the interface Comparator.

package de.vogella.algorithms.sort.standardjava;
 
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
 
public class Simple2 {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<Integer>();
        list.add(5);
        list.add(4);
        list.add(3);
        list.add(7);
        list.add(2);
        list.add(1);
        Collections.sort(list, new MyIntComparable());
        for (Integer integer : list) {
            System.out.println(integer);
        }
    }
}

The nice thing about this approach is that you then sort any object by any attribute or even a combination of attributes. For example if you have objects of type Person with an attribute income and dataOfBirth you could define different implementations of Comparator and sort the objects according to your needs.