Java，如何正确实现列表上的嵌套迭代器？

Question

I have two java List<String> objects, and I need to add in a third List<String> result object all the possibile combinations of the first two ones. Like:

List<String> list1 = getList1();
List<String> list2 = getList2();

List<String> result = new ArrayList<String>();

for(String value1 : list1) {
    for(String value2 : list2) {
        result.add(value1 + value2);
    }
}

The problem is that this quick and dirty function becomes exponentially slow when the lists grow:

long combinations = list1.size() * list2.size()

Maybe I'm doing it wrong, is there a better approach to it or any framework that can be used?

Answer 1

您可以通过调用result.size（）而不是将大小相乘得到想要的东西吗？

Answer 2

To make it faster, you can provide the initial capacity to the result so that the internal array of ArrayList is not extended:

int capacity = list1.size() * list2.size();
List<String> result = new ArrayList<String>(capacity);

Answer 3

The slightly-harder-but-definitely-better way is to extend an unmodifiable AbstractCollection (or AbstractList ) and the associated iterator (that will iterate on the two List iterators as needed). This way you won't need O(N*M) storage for the combined list.