Java中任意集的笛卡尔积 [英] Cartesian product of arbitrary sets in Java

问题描述

您是否知道一些简洁的Java库，允许您制作两个（或更多）集的笛卡尔积？

Do you know some neat Java libaries that allow you to make cartesian product of two (or more) sets?

例如：我有三套。一个是Person类的对象，第二个是Gift类的对象，第三个是GiftExtension类的对象。

For example: I have three sets. One with objects of class Person, second with objects of class Gift and third with objects of class GiftExtension.

我想生成一个包含所有可能三元组的集合Person-Gift- GiftExtension。

I want to generate one set containing all possible triples Person-Gift-GiftExtension.

集合的数量可能会有所不同，所以我不能在嵌套的foreach循环中执行此操作。
在某些情况下，我的应用程序需要制作人物 - 礼物对的产品，有时它是三重人物 - 礼品 - GiftExtension，有时可能甚至会设置Person-Gift-GiftExtension-GiftSecondExtension-GiftThirdExtension等。

The number of sets might vary so I cannot do this in nested foreach loop. Under some conditions my application needs to make a product of Person-Gift pair, sometimes it is triple Person-Gift-GiftExtension, sometimes there might even be sets Person-Gift-GiftExtension-GiftSecondExtension-GiftThirdExtension, etc.

推荐答案

编辑：删除了以前的两套解决方案。有关详细信息，请参阅编辑历史记录。

Previous solutions for two sets removed. See edit history for details.

以下是一种递归执行任意数量的集合的方法：

Here is a way to do it recursively for an arbitrary number of sets:

public static Set<Set<Object>> cartesianProduct(Set<?>... sets) {
    if (sets.length < 2)
        throw new IllegalArgumentException(
                "Can't have a product of fewer than two sets (got " +
                sets.length + ")");

    return _cartesianProduct(0, sets);
}

private static Set<Set<Object>> _cartesianProduct(int index, Set<?>... sets) {
    Set<Set<Object>> ret = new HashSet<Set<Object>>();
    if (index == sets.length) {
        ret.add(new HashSet<Object>());
    } else {
        for (Object obj : sets[index]) {
            for (Set<Object> set : _cartesianProduct(index+1, sets)) {
                set.add(obj);
                ret.add(set);
            }
        }
    }
    return ret;
}

请注意，无法使用返回的集保留任何泛型类型信息。如果您事先知道要使用多少个集合，则可以定义一个通用元组来保存那么多元素（例如 Triple< A，B，C> ），但是没有办法在Java中使用任意数量的泛型参数。

Note that it is impossible to keep any generic type information with the returned sets. If you knew in advance how many sets you wanted to take the product of, you could define a generic tuple to hold that many elements (for instance Triple<A, B, C>), but there is no way to have an arbitrary number of generic parameters in Java.

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