@Override
    public CompletionStage<Boolean> putIfAbsentAsync(K key, V value) {
        return cache.putIfAbsentAsync(key, value);
    }
@Test
    public void testPutIfAbsentAsync() throws Exception {
        cache.put(42, "oldValue");

        assertTrue(adapter.putIfAbsentAsync(23, "newValue").toCompletableFuture().get());
        assertFalse(adapter.putIfAbsentAsync(42, "newValue").toCompletableFuture().get());

        assertEquals("newValue", cache.get(23));
        assertEquals("oldValue", cache.get(42));
    }