### Question:
ConcurrentLongHashMap { public V computeIfAbsent(long key, LongFunction<V> provider) { checkNotNull(provider); long h = hash(key); return getSection(h).put(key, null, (int) h, true, provider); } ConcurrentLongHashMap();  ConcurrentLongHashMap(int expectedItems);  ConcurrentLongHashMap(int expectedItems, int concurrencyLevel); long size(); long capacity(); boolean isEmpty(); V get(long key); boolean containsKey(long key); V put(long key, V value); V putIfAbsent(long key, V value); V computeIfAbsent(long key, LongFunction<V> provider); V remove(long key); boolean remove(long key, Object value); int removeIf(LongObjectPredicate<V> predicate); void clear(); void forEach(EntryProcessor<V> processor); List<Long> keys(); List<V> values();  }### Answer:
@Test public void testComputeIfAbsent() { ConcurrentLongHashMap<Integer> map = new ConcurrentLongHashMap<>(16, 1); AtomicInteger counter = new AtomicInteger(); LongFunction<Integer> provider = new LongFunction<Integer>() { public Integer apply(long key) { return counter.getAndIncrement(); } }; assertEquals(map.computeIfAbsent(0, provider).intValue(), 0); assertEquals(map.get(0).intValue(), 0); assertEquals(map.computeIfAbsent(1, provider).intValue(), 1); assertEquals(map.get(1).intValue(), 1); assertEquals(map.computeIfAbsent(1, provider).intValue(), 1); assertEquals(map.get(1).intValue(), 1); assertEquals(map.computeIfAbsent(2, provider).intValue(), 2); assertEquals(map.get(2).intValue(), 2); }