@Override
  public boolean containsKey( Object key )            { return get(key) != null; }
Tests if the key in the table using the <tt>equals</tt> method.
@return <tt>true</tt> if the key is in the table using the <tt>equals</tt> method
@throws NullPointerException if the specified key is null
jctools-core/src/main/java/org/jctools/maps/NonBlockingIdentityHashMap.java