@Override
    public boolean add(E e) {
        return map.put(e, PRESENT) == null;
    }
Adds the specified element to this set if it is not already present. More
formally, adds the specified element <tt>e</tt> to this set if this set
contains no element <tt>e2</tt> such that
<tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>. If this
set already contains the element, the call leaves the set unchanged and
returns <tt>false</tt>.

@param e element to be added to this set
@return <tt>true</tt> if this set did not already contain the specified
element
core/common/src/main/java/com/alipay/sofa/rpc/common/struct/ConcurrentHashSet.java