class BestSolutionHolder<Solution_> {

    private final Lock problemChangesLock = new ReentrantLock();
    private final AtomicReference<VersionedBestSolution<Solution_>> versionedBestSolutionRef = new AtomicReference<>();
    private final SortedMap<BigInteger, List<CompletableFuture<Void>>> problemChangesPerVersion =
            new TreeMap<>();
    private BigInteger currentVersion = BigInteger.ZERO;

    boolean isEmpty() {
        return versionedBestSolutionRef.get() == null;
    }

    /**
     * NOT thread-safe.
     *
     * @return the last best solution together with problem changes the solution contains.
     */
    BestSolutionContainingProblemChanges<Solution_> take() {
        VersionedBestSolution<Solution_> versionedBestSolution = versionedBestSolutionRef.getAndSet(null);
        if (versionedBestSolution == null) {
            return null;
        }
        SortedMap<BigInteger, List<CompletableFuture<Void>>> containedProblemChangesPerVersion =
                problemChangesPerVersion.headMap(versionedBestSolution.getVersion().add(BigInteger.ONE));

        List<CompletableFuture<Void>> containedProblemChanges = new ArrayList<>();
        for (Map.Entry<BigInteger, List<CompletableFuture<Void>>> entry : containedProblemChangesPerVersion.entrySet()) {
            containedProblemChanges.addAll(entry.getValue());
            problemChangesPerVersion.remove(entry.getKey());
        }

        return new BestSolutionContainingProblemChanges<>(versionedBestSolution.getBestSolution(),
                containedProblemChanges);
    }

    /**
     * Sets the new best solution if all known problem changes have been processed and thus are contained in this
     * best solution.
     *
     * @param bestSolution the new best solution that replaces the previous one if there is any
     * @param isEveryProblemChangeProcessed a supplier that tells if all problem changes have been processed
     */
    void set(Solution_ bestSolution, BooleanSupplier isEveryProblemChangeProcessed) {
        problemChangesLock.lock();
        try {
            /*
             * The new best solution can be accepted only if there are no pending problem changes nor any additional
             * changes may come during this operation. Otherwise, a race condition might occur that leads to associating
             * problem changes with a solution that was created later, but does not contain them yet.
             * As a result, CompletableFutures representing these changes would be completed too early.
             */
            if (isEveryProblemChangeProcessed.getAsBoolean()) {
                versionedBestSolutionRef.set(new VersionedBestSolution(bestSolution, currentVersion));
                currentVersion = currentVersion.add(BigInteger.ONE);
            }
        } finally {
            problemChangesLock.unlock();
        }
    }

    /**
     * Adds a new problem change to a solver and registers the problem change to be later retrieved together with
     * a relevant best solution by the {@link #take()} method.
     *
     * @return CompletableFuture that will be completed after the best solution containing this change is passed to
     *         a user-defined Consumer.
     */
    CompletableFuture<Void> addProblemChange(Solver<Solution_> solver, ProblemChange<Solution_> problemChange) {<FILL_FUNCTION_BODY>}

    void cancelPendingChanges() {
        problemChangesLock.lock();
        try {
            problemChangesPerVersion.values()
                    .stream()
                    .flatMap(Collection::stream)
                    .forEach(pendingProblemChange -> pendingProblemChange.cancel(false));
            problemChangesPerVersion.clear();
        } finally {
            problemChangesLock.unlock();
        }
    }

    private static final class VersionedBestSolution<Solution_> {
        final Solution_ bestSolution;
        final BigInteger version;

        public VersionedBestSolution(Solution_ bestSolution, BigInteger version) {
            this.bestSolution = bestSolution;
            this.version = version;
        }

        public Solution_ getBestSolution() {
            return bestSolution;
        }

        public BigInteger getVersion() {
            return version;
        }
    }
}

        problemChangesLock.lock();
        try {
            CompletableFuture<Void> futureProblemChange = new CompletableFuture<>();
            problemChangesPerVersion.compute(currentVersion, (version, futureProblemChangeList) -> {
                if (futureProblemChangeList == null) {
                    futureProblemChangeList = new ArrayList<>();
                }
                futureProblemChangeList.add(futureProblemChange);
                return futureProblemChangeList;
            });
            solver.addProblemChange(problemChange);
            return futureProblemChange;
        } finally {
            problemChangesLock.unlock();
        }
    
<no_super_class>