Sorting an object in descending order using Array.sort and Lambda Expression

Question

I want to sort a collection of objects in descending order.

Below code is working to sort the itemDetails object in ascending order.

Arrays.sort(itemDetails, (a, b) -> 
    String.valueOf(a.getAvailableQuantity())
          .compareTo(String.valueOf(b.getAvailableQuantity())));

But I wanted the output in descending order.

Ori Marko · Accepted Answer · 2019-06-25 11:04:19Z

2

Switch to (b, a):

Arrays.sort(itemDetails, (b, a) ->
    String.valueOf(a.getAvailableQuantity())
          .compareTo(String.valueOf(b.getAvailableQuantity())));

Actually change names to meaningful so sorting logic will be understandable

edited Jun 25, 2019 at 11:04

answered Jun 25, 2019 at 10:44

Ori Marko

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2 Comments

Andy Turner Over a year ago

I would recommend against doing something as subtle as flipping the parameter order: it looks like a typo, TBH. If you're going to do something like this, I would reverse the order you use the arguments inside the lambda body.

Ori Marko Over a year ago

@AndyTurner added comment Actually change names to meaningful so sorting logic will be understandable

Andy Turner · Accepted Answer · 2019-06-25 19:13:31Z

2

Use Comparator.comparing (to avoid having to repeat the String.valueOf and getAvailableQuantity) and Comparator.reversed() (to clearly convey the fact that you are reversing the sort order):

Arrays.sort(
    itemDetails,
    Comparator.comparing((ItemDetails a) -> String.valueOf(a.getAvailableQuantity())).reversed());

where ItemDetails is the name of the element type of the array (or some supertype which also has the getAvailableQuantity method).

edited Jun 25, 2019 at 19:13

answered Jun 25, 2019 at 10:46

Andy Turner

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3 Comments

Shubhi Over a year ago

thanks but this is not working in my case giving me a error of typecasting of object.

Andy Turner Over a year ago

@Shubhi Add the type of the array element to the lambda parameter, e.g. (ItemDetail a) -> . Demo

Shubhi Over a year ago

I added it but that is giving me runtime error. @ Andy Turner

Maurice Perry · Accepted Answer · 2019-06-25 10:45:57Z

1

Add a minus:

Arrays.sort(itemDetails, (a, b) -> -String.valueOf(a.getAvailableQuantity()).compareTo(String.valueOf(b.getAvailableQuantity())));

edited Jun 25, 2019 at 10:45

answered Jun 25, 2019 at 10:44

Maurice Perry

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4 Comments

Andy Turner Over a year ago

What if compareTo had returned Integer.MIN_VALUE?

Maurice Perry Over a year ago

@AndyTurner it will not. But if that's a concern for you, you can also swap a and b.

Andy Turner Over a year ago

It might not in this specific case, so you can use this "one weird trick" here, but it should come with a health warning that you shouldn't do it in the general case.

Maurice Perry Over a year ago

@AndyTurner fair enough

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