public static <K, V> V computeIfAbsent(ConcurrentMap<K, V> map, K key, Supplier<V> supplier) {
    return computeIfAbsent(map, key, supplier, () -> {
    });
  }
In HBASE-16648 we found that ConcurrentHashMap.get is much faster than computeIfAbsent if the
value already exists. Notice that the implementation does not guarantee that the supplier will
only be executed once.
hbase-common/src/main/java/org/apache/hadoop/hbase/util/ConcurrentMapUtils.java