static void clear(byte[][] blocks, int off, int len) {
    // this is significantly faster than looping or Arrays.fill (which loops), particularly when
    // the length of the slice to be cleared is <= to ARRAY_LEN (in that case, it's faster by a
    // factor of 2)
    int remaining = len;
    while (remaining > ARRAY_LEN) {
      System.arraycopy(NULL_ARRAY, 0, blocks, off, ARRAY_LEN);
      off += ARRAY_LEN;
      remaining -= ARRAY_LEN;
    }

    System.arraycopy(NULL_ARRAY, 0, blocks, off, remaining);
  }
Clears (sets to null) all blocks between off (inclusive) and off + len (exclusive) in the given
array.
jimfs/src/main/java/com/google/common/jimfs/Util.java