@Override
	public Function<ISeq<Item>, Double> fitness() {
		return items -> {
			final Item sum = items.stream().collect(Item.toSum());
			return sum.size <= _knapsackSize ? sum.value : 0;
		};
	}
Create a new {@code Knapsack} definition with the given

@param items the basic {@link Set} of knapsack items.
@param knapsackSize the maximal knapsack size
@throws NullPointerException if the {@code items} set is {@code null}
jenetics.example/src/main/java/io/jenetics/example/Knapsack.java