How can I sort an ArrayList of arrays based on the array[0] element which is a date given in dd/mm/yyyy format?

I have a small issue in which I need to sort an arraylist of arrays based on the arrays 0th index. For example If I have an array list containing string arrays such as this:

arr1[0] = "12/21/2018" 
arr1[1] = "California"
arr[n] = "some string"

// and I have another array 
arr2[0] = "11/21/2018" 
arr1[1] = "California"
arr[n] = "some string". 

How would I go about sorting the arraylist based on the 0th index of each array in the array list?

Instead of dealing with putting an Array of Strings inside an ArrayList, why not just create a new class with properties? This class could implement Comparable so we can add a custom compare function that would allow us to sort the list in a custom fashion. For example, your class could be called Location, and has a LocalDate date (as mentioned by MadProgrammer), and a String name. An example would look a bit like this:

class Location implements Comparable<Location> {

// We implement the Comparable so we can Override the compareTo() method and make a custom
// sorting method.

	LocalDate date;
	String name;

	// Add more properties as needed...

	public Location(LocalDate d, String n) {
    // Constructor to help us create the object

		this.date = d;
		this.name = n;

	}

// I'm assuming you are comparing the String that was in the first index of the array as
// Dates in chronological order. That way we can compare them easily like so.
	@Override
	public int compareTo(Location other) {

		return this.date.compareTo(other.date);

	}

}

You could create an ArrayList of the Location class, which would be defined as something like this:

List<Location> locs = new ArrayList<>();

Then, you can sort it using Collections.sort(List<T> list). It sorts it using the comparator that was made in the class.

Java is Object Oriented. Use it to your advantage to avoid sticky situations like these.

Like Pulse I am using java.time, the modern Java date and time API:

List<String[]> arraylist = new ArrayList<>();
arraylist.add(new String[] { "12/21/2018", "California", "some string"});
arraylist.add(new String[] { "11/21/2018", "California", "some other string"});

DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern("MM/dd/uuuu");
Comparator<String[]> cmpr = Comparator.comparing(arr -> LocalDate.parse(arr[0], dateFormatter ));
arraylist.sort(cmpr);

arraylist.forEach(arr -> System.out.println(Arrays.toString(arr)));

Output with the example data given is:

> [11/21/2018, California, some other string]
> [12/21/2018, California, some string]

Link 1: Oracle tutorial: Date Time explaining how to use java.time.
Link 2: Answer by Pulse.

Here's a one-liner when data is as per title, ie dd/mm/yyyy:

Arrays.sort(arr, comparing(a -> a[0].replaceAll("(..).(..).(.*)", "$3$2$1")));

Here's a one-liner when data is as per example, ie mm/dd/yyyy:

Arrays.sort(arr, comparing(a -> a[0].replaceAll("(..).(..).(.*)", "$3$1$2")));