public Map<K, V> toConcurrentMap() {
        return inner.collect(Collectors.toConcurrentMap(
            Map.Entry::getKey,
            Map.Entry::getValue
        ));
    }
Accumulates all key-value pairs of this {@code MapStream} into a 
{@code ConcurrentMap}.
<p>
If the mapped keys contains duplicates (according to
{@link Object#equals(Object)}), an {@code IllegalStateException} is
thrown when the collection operation is performed. If the mapped keys
may have duplicates, use {@link #toConcurrentMap(BinaryOperator)} instead.

<p>
It is common for either the key or the value to be the input elements.
In this case, the utility method
{@link java.util.function.Function#identity()} may be helpful.
<p>
This is a {@link Collector.Characteristics#CONCURRENT concurrent} and
{@link Collector.Characteristics#UNORDERED unordered} Collector.

@return  a {@code ConcurrentMap} whose keys are taken from this stream

@see #toMap()
@see #toConcurrentMap(BinaryOperator)
common-parent/mapstream/src/main/java/com/speedment/common/mapstream/MapStream.java