Sort array in Java with lambda function

Question

Why did this lambda function work?

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

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

Why it's a.length() - b.length() , not just based on a.length()? Why does Arrays.sort(months,(String a) -> a.length()) not work?;

Answer 1

The (String a, String b) -> a.length() - b.length() is the Single-Abstract-Method shortcut for Comparator , and Comparator 's compare method must satisfy compare(a, b) > 0 if and only if compare(b, a) < 0 , otherwise it wouldn't define a proper comparison function. Such a function obviously cannot work if you discarded the argument b and returned an int based only on a . If you wanted to "sort-by" length, you could use Comparator.comparing(String::length) (see here ).