@Override
    public Collection<R> values() {
      return resources;
    }
The <code>RoundRobinPool</code> represents a {@link PoolMap.Pool}, which stores its resources
in an {@link ArrayList}. It load-balances access to its resources by returning a different
resource every time a given key is looked up.
<p>
If {@link #maxSize} is set to {@link Integer#MAX_VALUE}, then the size of the pool is
unbounded. Otherwise, it caps the number of resources in this pool to the (non-zero positive)
value specified in {@link #maxSize}.
</p>
@param <R> the type of the resource
hbase-client/src/main/java/org/apache/hadoop/hbase/util/PoolMap.java