Comparator<T> example

gistfile1.java

import java.util.List;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;

public class Test
{
	public static void main(String[] args)
	{
		Integer[] numbers = { 1, 5, 2, 7, 2, 42, 1 };
		
		// sort ascending
		Arrays.sort(numbers, new Comparator<Integer>() {
			public int compare(Integer a, Integer b)
			{	
				// if a > b, then result will be positive -> a after b
				// if b > a, then result will be negative -> b after a
				return a - b;
			}
		});
		System.out.println("Ascending:");
		for (int i = 0; i < numbers.length; i++)
			System.out.println(numbers[i]);

		// sort descending
		Arrays.sort(numbers, new Comparator<Integer>() {
			public int compare(Integer a, Integer b)
			{
				return b - a; // reversed
			}
		});
		System.out.println("\nDescending:");
		for (int i = 0; i < numbers.length; i++)
			System.out.println(numbers[i]);

		// also works for List<...> type (or ArrayList, LinkedList, etc.), 
		// using Collections.sort(...) instead of Arrays.sort(...)
		List<Integer> numberList = Arrays.asList(numbers);
		Collections.sort(numberList, new Comparator<Integer>() {
			public int compare(Integer a, Integer b)
			{
				return a - b;
			}
		});
		System.out.println("\nSorted List<Integer>:");
		for (Integer n : numberList)
			System.out.println(n);

		Integer[][] lists = {
			{ 2, 4, 3 },
			{ 32, 34, 1, 54, 3, 1 },
			{ 2, 4, 3, 6, 1 },
			{ 1, 4 }
		};

		// sort by list length
		Arrays.sort(lists, new Comparator<Integer[]>() {
			public int compare(Integer[] a, Integer[] b)
			{
				// if length of list a > length of listb, 
				// 	then result will be positive -> a after b
				// if length of list b > length of list a, 
				// 	then result will be negative -> b after a
				return a.length - b.length;
			}
		});
		System.out.println("\nSorted by list length:");
		for (int i = 0; i < lists.length; i++)
			System.out.println("List of length " + lists[i].length);
	}
}