java: Arrays.sort() with lambda expression

Question

I want to sort String elements in the array months by length using Arrays.sort method. I was told here, that it's possible to use lambda expressions instead of creating new class implementing Comparator. Did it exactly the same way, yet it doesn't work.

import java.util.Arrays;
import java.util.Comparator;

public class MainClass {
public static void main(String[] args)
{

    String[] months = {"January","February","March","April","May","June","July","August","September","October","December"};

    System.out.println(Arrays.toString(months)); //printing before


    //neither this works:
    Arrays.sort(months, 
            (a, b) -> Integer.signum(a.length() - b.length())   
    );

    //nor this:
    Arrays.sort(months, 
            (String a, String b) -> { return Integer.signum(a.length() - b.length()) }; 
    );


    System.out.println(Arrays.toString(months)); //printing after
}
}

Solution 1

The cleanest way would be:

Arrays.sort(months, Comparator.comparingInt(String::length));

or, with a static import:

Arrays.sort(months, comparingInt(String::length));

---

However, this would work too but is more verbose:

Arrays.sort(months,
            (String a, String b) -> a.length() - b.length());

Or shorter:

Arrays.sort(months, (a, b) -> a.length() - b.length());

---

Finally your last one:

Arrays.sort(months, 
    (String a, String b) -> { return Integer.signum(a.length() - b.length()) }; 
);

has the ; misplaced - it should be:

Arrays.sort(months, 
    (String a, String b) -> { return Integer.signum(a.length() - b.length()); }
);

Solution 2

You're looking for this:

Arrays.sort(months, (a, b) -> Integer.signum(a.length() - b.length()));

Solution 3

The functionality you are looking for is in Java 8, which has not yet been released. It is scheduled for release in a few months if you want to wait, or if not beta downloads are available.

Comments

[skiwi]

+1 for showing Comparator.comparing, it is so much easier to grasp than writing a custom lambda or Comparator.

[matt-pielat]

Well, that's helpful, but I'd like to know what's wrong with my solution.

[Josh M]

Works for me... Are you sure you're using Java 8? Which IDE are you using?

[assylias]

@lavsprat I have added something showing your syntax error.

[skiwi]

@lavsprat It also works for me, it prints [May, June, July, March, April, August, January, October, February, December, September]... What Java 8 version are you using?

[Vishy]

I suggest using Integer.compare instead of signum of -.

[assylias]

@PeterLawrey Indeed - signum(a-b) has the risk of overflowing (even if it's unlikely for strings).

[assylias]

@lavsprat Then you don't have java 8 installed or you are not compiling with the right JDK or your IDE does not support it or you have a very old version or...

[matt-pielat]

I'm using Eclipse. This site says I have Version 7 Update 51.

[matt-pielat]

Oracle site says "You have the recommended Java installed (Version 7 Update 51).".

[assylias]

@lavsprat huh.... Version 7 != Version 8... Java 8 is here: jdk8.java.net/download.html

[Josh M]

Well of course you can't use lambdas then, as they are only available in Java 8.

[Stuart Marks]

I don't think - can overflow in this case, as both values are non-negative.

[Ted Hopp]

Useful comment, but not an answer.

[Tim B]

@TedHopp It is an answer. His code isn't compiling because he's trying to use a Java 8 feature in Java 7!

[Ted Hopp]

His code isn't compiling because it has a syntax error (semicolon at the end of the second argument to Arrays.sort).

[Holger]

Integer.signum is obsolete. Just returning a-b (for these special a and b where a-b cannot overflow, of course) is enough. There is no need to set the absolute value to 1. So wrapping a value which has the right sign already with an Integer.signum(…) call makes no sense for a Comparator.

[Maurice Naftalin]

This comment is a bit misleading, unless you count three weeks as being "a few months": ship date is March 18th

[Ole V.V.]

I certainly prefer Comparator.comparingInt() over comparing() when what you are comparing are indeed ints. It’s clearer (and also avoids unnecessary boxing).