使用Java8匹配方法比较两个列表

Question

Using Java8 matching methods am able to compare two list and getting the boolean results when if the is any match is available in both the lists.

Please find my below code for that.

public class StreamTest2 {

    public static void main(String args[]) {

        List<Integer> aList = Arrays.asList( new Integer[] {
                1,3,5,6,8
        });

        List<Integer> bList = Arrays.asList( new Integer[] {
                10, 89, 8, 9
        });

        //If any number in List1 present in List2
        System.out.println("If any number present in aList is present in bList : "+aList.stream().anyMatch(num -> bList.contains(num)));

    }

}

Output :

If any number present in aList is present in bList : true

But, i want to print the matching number from both the list, how i can print the matching number ?

Answer 1

You can use filter and findFirst :

System.out.println("If any number present in aList is present in bList : "+aList.stream().filter(num -> bList.contains(num)).findFirst().orElse(null));

This will print the matching number if found (it will stop at the first match), or null , if no match is found.

Or, you can collect all the matches into a List :

System.out.println("If any number present in aList is present in bList : "+aList.stream().filter(num -> bList.contains(num)).collect(Collectors.toList());

Answer 2

试试这个

aList.stream().filter(bList::contains).collect(Collectors.toSet());

Answer 3

The reason because of which you get java.lang.UnsupportedOperationException when you call retainAll is that Arrays#asList returns an ArrayList backed by an array of fixed size. Any attempt to remove or add an element to these lists will result in the aforementioned UnsupportedOperationException .

The solution is, as @LuCio has suggested, to wrap the Arrays.asList part with a ArraysList constructor call, as such:

List<Integer> aList = new ArrayList(Arrays.asList(1,3,5,6,8));
List<Integer> bList = new ArrayList(Arrays.asList(10, 89, 8, 9));

the you should be able to call:

aList.retainAll(bList);

Keep in mind that this will modify the original aList list. If you need to preserve the state of aList then I would suggest to make a copy of aList before invoking retainAll . One of the suggested Stream API approached would in this case maybe be more suitable.