The spec states that a deque created with the default constructor should be able to hold 16 elements.

https://docs.oracle.com/javase/10/docs/api/java/util/ArrayDeque.html#%3Cinit%3E()
"""
Constructs an empty array deque with an initial capacity sufficient to hold 16 elements.
"""

In the constructor an array of size 16 is created:
        elements = new Object[16];

However, one element is always null:
    /**
     * The array in which the elements of the deque are stored.
     * All array cells not holding deque elements are always null.
     * The array always has at least one null slot (at tail).
     */
    transient Object[] elements;

which leaves us space for only 15 elements, contrarily to what the spec promises.