Collection.retainAll（）匹配问题

Question

I am attempting to find the intersection between multiple collections of numbers.
For example, I want to find the intersection between the following 3 arrays of numbers:
a = {0,1,1}
b = {1,1,2}
c = {0,1,2}
The result should be:
intersection(a,b,c) = {1}
but when I do it [sequentially] (see included code below) I get: intersection = {1,1}. What should I do differently such that I can get my desired result?

Thanks,
MGR

ArrayList<Integer> a = new ArrayList<>(Arrays.asList(new Integer[] {0,1, 1}));
    ArrayList<Integer> b = new ArrayList<>(Arrays.asList(new Integer[]{2, 1, 1}));
    ArrayList<Integer> c = new ArrayList<>(Arrays.asList(new Integer[]{0,2,1}));

    ArrayList<Integer> intersection = a;
    /*System.out.println("intersection is "+intersection);
    System.out.println("b is\t"+b);
    System.out.println("A and B");*/
    //System.out.println(intersection.retainAll(b));
    intersection.retainAll(b);
    System.out.println("intersection now is "+intersection);
    /*System.out.println(intersection==null);
    System.out.println(intersection.isEmpty());
    System.out.println("Doing c:");
    System.out.println("c is\t"+c);
    System.out.println(intersection.retainAll(c));*/
    intersection.retainAll(c);
    System.out.println("intersection is "+intersection);

Answer 1

使用Set作为交集而不是List ：

Set<Integer> intersection = new HashSet<>(a);