Question

So I have this method which will take an ArrayList of Integers, I compare between 2 sub arrays and preform some swapping of numbers.

I am comparing the first elements of each sub array which is just the array halved, then swapping the elements if the bigger number isn’t in the left subList. after that I want to recursively do the same thing but to the first half.

so if I type 3 5 8 2 1 7 6 4
I want to see this;

[3, 5, 8, 2, 1, 7, 6, 4]

[3, 7, 8, 4, 1, 5, 6, 2]

[8, 7, 3, 4, 1, 5, 6, 2]

[8, 7, 3, 4, 1, 5, 6, 2]

What I want to be able to do is once I have done what I need to, to the base array, I need to work on the first half of the same array recursively. However when I compile, I get an error saying that I have incompatible types. So I’m assuming the subList function isn’t providing me a ArrayList.

Any pointers on gettign this to work correctly?

public static void tournament(ArrayList<Integer> players){
int mid = players.size()/2;
    for(int i=0; i < mid; i++){
        if(players.subList(0,mid).get(i) < players.subList(mid,players.size()).get(i)){
            int temp = players.subList(0,mid).get(i);       
            players.subList(0,mid).set(i , players.subList(mid,players.size()).get(i));
            players.subList(mid,players.size()).set(i,temp);    
        }// end if
    }// end for
    System.out.println(players);

    if(players.size() > 2){
        tournament(players);
    }// end if

}// end tournament

Answer 1

However when I compile, I get an error saying that I have incompatible types. So I’m assuming the subList function isn’t providing me a ArrayList.

With the Collections API (of which the ArrayList is part), the best-practice is to declare your variables using the generic interfaces: List, Set, Map, rather than their concrete implementations such as ArrayList, HashSet and HashMap.

For example, instead of this:

ArrayList<Integer> list = new ArrayList<Integer>();

you should do this:

List<Integer> list = new ArrayList<Integer>();

If you take a look at the JavaDoc for java.util.List.subList(), you’ll see that the method signature is this:

List<E> subList(int fromIndex, int toIndex)

ArrayList, as an implementation of the List interface, uses this exact same signature. Note that the return type is List, and not ArrayList. What this means is that ArrayList.subList() doesn’t have to return you an ArrayList (although it could) – it simply has to return you an object that conforms to the List interface.

So what is the first step? Change your method to accept a List rather than an ArrayList. (Whatever method calls your tournament() initially method can still pass an ArrayList.)

public static void tournament(List<Integer> players)