public int nextIndex() {
		int index = -1;
		for (int i = 0; i <  _values.length && index == -1; ++i) {
			if (Double.isNaN(_values[i])) {
				index = i;
			}
		}

		return index;
	}
Return the index of the next free {@code double} <i>slot</i>. If all
<i>slots</i> are occupied, {@code -1} is returned.

@return the index of the next free {@code double} <i>slot</i>, or
        {@code -1} if all <i>slots</i> are occupied
jenetics.tool/src/main/java/io/jenetics/tool/trial/Sample.java