public class CollectionTest {
 
    public static void main(String[] args) {
        List<String> list1 = new ArrayList<>();
        List<String> list2 = new ArrayList<>();
 
        list1.add("a");
        list1.add("b");
        list1.add("c");
 
        list2.add("b");
        list2.add("e");
        list2.add("d");
 
        //求交集
        list1.retainAll(list2);
        System.out.println(list1.toString()); // b
 
        //求并集
        list1.removeAll(list2);
        list1.addAll(list2);
        System.out.println(list1.toString());  // a c b e d
 
        //求差集(list1有而list2沒(méi)有)
        list1.removeAll(list2);   // a c
 
    }
}

public List<Object> intersectionForList_1(List<Object> arr1, List<Object> arr2) {
    long startTime = System.currentTimeMillis();
    List<Object> result = new ArrayList<>();
    for (Object arr : arr1) {
        if (arr2.contains(arr)) {
            result.add(arr);
         }
    }
    long endTime = System.currentTimeMillis();
    log.info("intersectionForList_1：" + (endTime - startTime));
    return result;
}

Set<Integer> set1 = new HashSet<>();
set1.add(1);
set1.add(2);
set1.add(3);
set1.add(4);

Set<Integer> set2 = new HashSet<>();
set2.add(3);
set2.add(4);
set2.add(5);
Set<Integer> intersection = set1.stream().filter(set2::contains).collect(Collectors.toSet());
System.out.println(intersection); // 輸出 [3, 4]

public List<Object> intersectionForList_2(List<Object> arr1, List<Object> arr2) {

      List<Object> resultList = new ArrayList<>();
 
      List<Object> maxList;
      List<Object> minList;
      if (arr1.size()>arr2.size()){
          maxList = arr1;
          minList = arr2;
      }else {
          maxList = arr2;
          minList = arr1;
      }
 
      Iterator<Object> iterator = maxList.iterator();
      while (iterator.hasNext()){
          Object next = iterator.next();
          if (minList.contains(next)){
              resultList.add(next);
          }
      }
 

      return resultList;
  }

public List<Object> intersectionForList_3(List<Object> arr1, List<Object> arr2) {

    List<Object> resultList = new ArrayList<>();
    Map<String,Object> map = new HashMap<>();
    arr1.forEach(a1->{
         map.put(a1+"",a1);
    });
 
    arr2.forEach(a2->{
        Object obj = map.get(a2 + "");
        if (obj!=null){
            resultList.add(obj);
        }
    });

    return resultList;
}