Java Compare Two Lists

Question

I have two lists ( not java lists, you can say two columns)

For example

**List 1**            **Lists 2**
  milan                 hafil
  dingo               


        

Answer 1

Are these really lists (ordered, with duplicates), or are they sets (unordered, no duplicates)?

Because if it's the latter, then you can use, say, a java.util.HashSet and do this in expected linear time using the convenient retainAll.

    List list1 = Arrays.asList(
        "milan", "milan", "iga", "dingo", "milan"
    );
    List list2 = Arrays.asList(
        "hafil", "milan", "dingo", "meat"
    );

    // intersection as set
    Set intersect = new HashSet(list1);
    intersect.retainAll(list2);
    System.out.println(intersect.size()); // prints "2"
    System.out.println(intersect); // prints "[milan, dingo]"

    // intersection/union as list
    List intersectList = new ArrayList();
    intersectList.addAll(list1);
    intersectList.addAll(list2);
    intersectList.retainAll(intersect);
    System.out.println(intersectList);
    // prints "[milan, milan, dingo, milan, milan, dingo]"

    // original lists are structurally unmodified
    System.out.println(list1); // prints "[milan, milan, iga, dingo, milan]"
    System.out.println(list2); // prints "[hafil, milan, dingo, meat]"