FULL PRODUCT VERSION :
java version "1.8.0_92"
Java(TM) SE Runtime Environment (build 1.8.0_92-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.92-b14, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Darwin lynx.local 16.0.0 Darwin Kernel Version 16.0.0: Mon Aug 29 17:56:20 PDT 2016; root:xnu-3789.1.32~3/RELEASE_X86_64 x86_64


A DESCRIPTION OF THE PROBLEM :
Calling clear() on a ConcurrentSkipListSet while also calling add() can cause the set to become unusable.

It may appear isEmpty() with size()==0, yet still contain elements per contains() and add().

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the test program noted below.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The program should not terminate.
ACTUAL -
$ java TestSet
**** An empty set contains 'c'! (Iteration 79)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ConcurrentSkipListSet;

public class TestSet {
    public static void main(String[] args) {
        int i = 0;
        while (true) {
            i++;
            ConcurrentSkipListSet<String> set = new ConcurrentSkipListSet<>();
            set.add("a");
            CompletableFuture<Void> future = CompletableFuture.runAsync(
                    () -> set.clear());
            set.add("b");
            set.add("c");
            future.join();
            if (set.isEmpty() && set.contains("c")) {
                System.out.println(
                        "**** An empty set contains 'c'! (Iteration " + i + ")");
                break;
            }
        }
    }
}

---------- END SOURCE ----------