public boolean contains( final int i ) {
      if( (i>>6) >= _bits.length ) // Out of bounds?  Not in this array!
        return _new != null && help_copy().contains(i);

      // Handle every 64th bit via using a nested array
      NBSI nbsi = this;         // The bit array being added into
      int j = i;                // The bit index being added
      while( (j&63) == 63 ) {   // Bit 64? (low 6 bits are all set)
        nbsi = nbsi._nbsi64;    // Recurse
        j = j>>6;               // Strip off low 6 bits (all set)
      }

      final long mask = mask(j);
      long old = nbsi._bits[j>>6]; // Read old bits
      if( old < 0 )             // Not mutable?
        // Not mutable: finish copy of word, and retry on copied word
        return help_copy_impl(i).help_copy().contains(i);
      // Yes mutable: test & return bit
      return (old & mask) != 0;
    }
Standard Java {@link Iterator}.  Not very efficient because it
auto-boxes the returned values.
jctools-core/src/main/java/org/jctools/maps/NonBlockingSetInt.java