Stream.dropWhile() doesn't return correct value in two different values

Question

I am trying to learn new features in Java-9 I come to know about the dropWhile method of Stream but it is returning different values in two different scenarios. Here is my code

package src.module;

import java.util.stream.Collectors;
import java.util.stream.Stream;
import java.util.List;

public class Test {

    public static void main(String[] args) {

        String s[] = new String[3];
        s[0] = "Hello";
        s[1] = "";
        s[2] = "World";
        List<String> value = Stream.of(s).dropWhile(a -> a.isEmpty()).collect(Collectors.toList());

        System.out.println(value);

        List<String> values = Stream.of("a", "b", "c", "", "e", "f").dropWhile(d -> !d.isEmpty())
                .collect(Collectors.toList());
        System.out.println(values);

    }
}

Here is the answer what I am getting

[Hello, , World]
[, e, f]

What I think in first condition it should print [,World]. Thanks in advance.

Answer 1

The dropWhile method, introduced in Java 9, will remove the longest starting set of elements that match the predicate.

Returns, if this stream is ordered, a stream consisting of the remaining elements of this stream after dropping the longest prefix of elements that match the given predicate.

Because your condition is that the item is empty, and the first item is not empty, nothing is removed, leaving ["Hello", "", "World"] intact.

At the end, when you call dropWhile with the opposite condition, is not empty, the first 3 items match and are removed, leaving ["", "e", "f"], which are the remaining items.

This is the expected behavior.

Answer 2

Your first condition is saying to drop items until a non-empty item is found The second condition says to drop items until an empty item is found. Add a '!' to your first condition to get your predicted result.