public abstract boolean removeFirstOccurrence (Object o)

Removes the first occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first element e such that (o==null ? e==null : o.equals(e)) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).

o    element to be removed from this deque, if present

Returns:  true if an element was removed as a result of this call

ClassCastException    if the class of the specified element is incompatible with this deque (optional)
NullPointerException    if the specified element is null and this deque does not permit null elements (optional)