How do I combine a subList and a Hashset (ArrayList)?

Question

I am making an ArrayList with 3 different types of groups. The objective I am trying to achieve is:

1. It ensures randomization (Collections.shuffle) (Done)
2. Using a sublist, I can pick a specific number of elements in the ArrayList (Done)
3. To prevent duplication appearing in the other groups when running the program, I implement a HashSet (Not done)

I can't seem to combine #2 and #3. Maybe I'm just not understanding how HashSets are done yet. Here is my code:

class code {
    public static void main(String[] args) {

    //This is creating the ArrayList with everything added

    List one = new ArrayList();
    List two = new ArrayList();
    List three = new ArrayList();

    one.add("United States");
    one.add("United Kingdom");
    one.add("Italy");
    one.add("France");
    one.add("Russia");
    one.add("Japan");
    one.add("China");
    one.add("Mexico");

    two.add("Philippines");
    two.add("Austria");
    two.add("Canada");
    two.add("Sweden");
    two.add("Iceland");
    two.add("India");
    two.add("Australia");
    two.add("Deutschland");

    three.add("Norway");
    three.add("Kosovo");
    three.add("North Korea");
    three.add("Sudan");
    three.add("United Arab Emirates");
    three.add("Bahrain");
    three.add("Haiti");

    //Formatting purposes

    System.out.println("Country Categories: ");
    System.out.println("#1 Countries = " + one);
    System.out.println("#2 Countries = " + two);
    System.out.println("#3 Countries = " + three);
    System.out.println("\n");

    //Randomization code. I use this method so it's shuffled the same way.

    long seed = System.nanoTime();
    Collections.shuffle(one, new Random(seed));
    Collections.shuffle(two, new Random(seed));
    Collections.shuffle(three, new Random(seed));

    System.out.println("Group #1");
    List largelist = one.subList(1,4);
    System.out.println(largelist);
    List mediumlist = two.subList(1,4);
    System.out.println(mediumlist);
    List smalllist = three.subList(1,3);
    System.out.println(smalllist);

    /*System.out.println("Group #2");
    List largerlist = one.subList(1,4);
    System.out.println(largerlist);
    List mediumrlist = two.subList(1,4);
    System.out.println(mediumrlist);
    List smallrlist = three.subList(1,3);
    System.out.println(smallrlist);

    System.out.println("Group #3");
    List largerrlist = one.subList(1,3);
    System.out.println(largerrlist);
    List mediumrrlist = two.subList(1,3);
    System.out.println(mediumrrlist);
    List smallrrlist = three.subList(1,3);
    System.out.println(smallrrlist);*/
    }
} //I only put one group because of issue I am having

I didn't implement my HashSet in this because it looked embarrassing but here it is:

System.out.println("Group #1");
Set largeset = new HashSet(one);
for (Object wow : one) {
  List largelist = one.subList(1,4);
  System.out.println(largelist);
  System.out.println(wow);

My intended output I wanted is something like this:

Group #1 //Week 1
[United Kingdom, Italy, Russia]
[Philippines, Canada, Iceland]
[Bahrain, Sudan]

Group #2
[France, United States, Japan]
[Sweden, India, Austria]
[Kosovo, North Korea]

Group #3
[Mexico, China]
[Australia, Deutschland]
[United Arab Emirates, Norway]

Randomization to Randomize, SubList to choose specific arrays and print them, and a HashSet to prevent duplication.

Keep in mind this is just for 1 week, now I must find a way for it to recur so that every country meets up at least once in 5 weeks -_- Help/Advice/Examples are appreciated, but figuring this out is my main goal.

Answer 1

If the type of the members in the list is just String, your code works well and I test it. But if you want to avoid duplicate self-defined class, you will need to override the equals() and hashCode() method. You should read up on how to ensure that you've implemented equals and hashCode properly. This is a good starting point: What issues should be considered when overriding equals and hashCode in Java?

Answer 2

private static Set allCountries = new HashSet();

private static void distinctAdd(String country, List targetList){
    if(!allCountries.contains(country)){
        allCountries.add(country);
        targetList.add(country);
    }else{
        //don't add duplicates!
    }
}

public static void main(String args[]) {

    List one = new ArrayList();
    List two = new ArrayList();
    List three = new ArrayList();

    distinctAdd("United States", one);
    distinctAdd("United Kingdom", one);
    distinctAdd("Italy", one);
    distinctAdd("France", one);
    distinctAdd("Russia", one);
    distinctAdd("Japan", one);
    distinctAdd("China", one);
    distinctAdd("Mexico", one);

    distinctAdd("Philippines", two);
    distinctAdd("Austria", two);
    distinctAdd("Canada", two);
    distinctAdd("Sweden", two);
    distinctAdd("Iceland", two);
    distinctAdd("India", two);
    distinctAdd("Australia", two);
    distinctAdd("Deutschland", two);
    distinctAdd("United States", two);

    distinctAdd("Norway", three);
    distinctAdd("Kosovo", three);
    distinctAdd("North Korea", three);
    distinctAdd("Sudan", three);
    distinctAdd("United Arab Emirates", three);
    distinctAdd("Bahrain", three);
    distinctAdd("Haiti", three);

    System.out.println("Country Categories: ");
    System.out.println("#1 Countries = " + one);
    System.out.println("#2 Countries = " + two);
    System.out.println("#3 Countries = " + three);
    System.out.println("\n");

}