I will be happy if someone can explain why I can not compare String in an array using the Stream API in Java.
I am trying to filter only the pairs when the first word is before the second lexicographically.
String[] input = { "Apple", "Banana" };
Arrays.stream(input)
.filter( (a,b)-> a.compareTo(b)< 0 )
It seems that Java doesn't understand, that "b" is a String, but why?
filter expects a Predicate<T> (or to be more exact, Predicate<? super T>), where T is the type of the Stream element (String in your case).
(a,b)-> a.compareTo(b)< 0 cannot be interpreted as a Predicate<T>, since the Predicate's boolean test(T t) method takes just one argument, not two.
filter is applied on each element of a Stream separately. There are no pairs in your Stream.
If you have more pairs to compare, this could be a help:
public class Main {
public static void main(String[] args) {
String[][] input = {{ "Apple", "Banana" },
{"Orange", "Apple"},
{"Banana", "Orange"}};
Arrays.stream(input)
.filter(x -> x[0].compareTo(x[1]) < 0)
.forEach(x -> System.out.println(Arrays.toString(x)));
}
}
And now the output should look like:
[Apple, Banana]
[Banana, Orange]
Hope it helps you with your problem.
If you want to compare pairs, you need to "make" the pairs yourself somehow, e.g.
IntStream indexes =
IntStream.range(0, input.length-1)
// Via i you implicitly have access to the pair
// (input[i], input[i+1])
.filter(i -> input[i].compareTo(input[i+1]) < 0);
This yields the indexes of elements lexicographically before their following element.
You can do further operations on this stream; but it's not entirely clear what result you expect.
