Java Comparator class to sort arrays

Question

Say, we have the following 2-dimensional array:

int camels[][] = new int[n][2];

How should Java Comparator class be declared to sort the arrays by their first elements in decreasing order using Arrays.sort(camels, comparator)? The compare function for reference is:

@Override public int compare(int[] a, int [] b)
{
    return b[0] - a[0];
}

Answer 1

[...] How should Java Comparator class be declared to sort the arrays by their first elements in decreasing order [...]

Here's a complete example using Java 8:

import java.util.*;

public class Test {

    public static void main(String args[]) {

        int[][] twoDim = { {1, 2}, {3, 7}, {8, 9}, {4, 2}, {5, 3} };

        Arrays.sort(twoDim, Comparator.comparingInt(a -> a[0])
                                      .reversed());

        System.out.println(Arrays.deepToString(twoDim));
    }
}

Output:

[[8, 9], [5, 3], [4, 2], [3, 7], [1, 2]]

---

For Java 7 you can do:

Arrays.sort(twoDim, new Comparator<int[]>() {
    @Override
    public int compare(int[] o1, int[] o2) {
        return Integer.compare(o2[0], o1[0]);
    }
});

---

If you unfortunate enough to work on Java 6 or older, you'd do:

Arrays.sort(twoDim, new Comparator<int[]>() {
    @Override
    public int compare(int[] o1, int[] o2) {
        return ((Integer) o2[0]).compareTo(o1[0]);
    }
});

Answer 2

The answer from @aioobe is excellent. I just want to add another way for Java 8.

int[][] twoDim = { { 1, 2 }, { 3, 7 }, { 8, 9 }, { 4, 2 }, { 5, 3 } };

Arrays.sort(twoDim, (int[] o1, int[] o2) -> o2[0] - o1[0]);

System.out.println(Arrays.deepToString(twoDim));

For me it's intuitive and easy to remember with Java 8 syntax.

Answer 3

Just tried this solution, we don't have to even write int.

int[][] twoDim = { { 1, 2 }, { 3, 7 }, { 8, 9 }, { 4, 2 }, { 5, 3 } };
Arrays.sort(twoDim, (a1,a2) -> a2[0] - a1[0]);

This thing will also work, it automatically detects the type of string.

Answer 4

java.nio.IntBuffer#wrap(int[]) provides an excellent built-in way to compare two instances of int[], since IntBuffer is both a lightweight wrapper for int[] instances, and implements Comparable. Using it combined with other built-in Comparator features has several advantages over the examples in other answers I see here:

• Compares all sub-array elements
• Supports variable sub-array lengths
• Supports null array elements

This example sorts the arrays in descending order, putting null array elements last:

int[][] twoDim = {{1, 2}, {3, 7}, {8, 9}, {4, 2}, null, {5, 3}, {4}};
System.out.println("Unsorted: " + Arrays.deepToString(twoDim));

Comparator<int[]> c = Comparator.nullsFirst(Comparator.comparing(IntBuffer::wrap));
Arrays.sort(twoDim, c.reversed());
System.out.println("Sorted: " + Arrays.deepToString(twoDim));

Output:

Unsorted: [[1, 2], [3, 7], [8, 9], [4, 2], null, [5, 3], [4]]
Sorted: [[8, 9], [5, 3], [4, 2], [4], [3, 7], [1, 2], null]

Answer 5

Here's sorting Arraylist of Date Arrays. Maybe smb smwhen will need it.

List <Date[]> sortedDateList = new ArrayList<>(/* initialization */);
Collections.sort(sortedDateList, new Comparator<Date[]>() {
    @Override
    public int compare(Date[] d1, Date[] d2) {
        return (d1[0].compareTo(d2[0]));
    }
});