Returns the greatest key strictly less than the given key, or {@code null} if there is no such key.
@param key the key
@return the greatest key less than {@code key}, or {@code null} if there is no such key
@throws ClassCastException if the specified key cannot be comparedwith the keys currently in the map
@throws NullPointerException if the specified key is nulland this map does not permit null keys
@throws ClassCastException {@inheritDoc}
@throws NullPointerException if the specified key is nulland this map uses natural ordering, or its comparator does not permit null keys
@since 1.6
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