import java.util.*;
import java.util.stream.*;
 
 
class Ideone {
    private static Optional<Integer> transform(final int i) {
        return (i < 0) ? Optional.ofNullable(null) : Optional.of(i);
    }
 
    public static void main(final String[] arguments) {
        final Random rand = new Random();
 
        final List<Integer> integers = Stream.generate(rand::nextInt).map(i -> i % 100).limit(10).collect(Collectors.toList());
        final List<Integer> valid = new ArrayList<Integer>();
        for(final int i : integers) {
            final Optional<Integer> transformed = transform(i);
            if(transformed.isPresent())
                valid.add(i);
            else
                System.out.println("[Error] Negative number: " + i);
        }
        System.out.println("Valid numbers: [" + String.join(", ", valid.stream().map(Object::toString).collect(Collectors.toList())) + "]");
    }
}
 
 

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