How to make a selection sort algorithm using Java collection linkedlist?

Question

I'm new to Java and I need to make a selection sort algorithm using Java LinkedList. I tried making the insertion sort, but I can't turn it into a selection sort. This is my selection sort code:

import java.util.*;
public class select {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        LinkedList<Integer> data = new LinkedList<>();

        System.out.println("Enter total count of elements-> ");
        int num = input.nextInt();

        while(num>0){
            data.add(input.nextInt());
            num--;
        }

        System.out.println("Original data:\n" +data);

        LinkedList<Integer> sortedData=sort(data);
        System.out.println("Sorted data:\n"+sortedData);
    }
    
    public static LinkedList<Integer> sort(LinkedList<Integer> data) {
        
        ListIterator<Integer> iterator=data.listIterator();
        while (iterator.hasNext()) {
            int key=iterator.next();
            

            for(int i = 0 ;i<data.size()-1;i++) {
                for(int j = i+1; j < data.size(); j++){
                    if(data.get(j) < key){
                        int x = data.get(j);
                        int y = key;
                        swap(data, x, y);
                    }
                }
                
            }
        }
        return data;
    }

    private static void swap(LinkedList<Integer> data, int x, int y) {
        int index1 = data.indexOf(x);
        int index2 = data.indexOf(y);

        if(index1 == -1 || index2== -2){
            return;
        }
    }
}

The sorted data is always the same as the original data and I don't know which went wrong.

Answer 1

The problem seems to be that your swap method doesn't actually swap the values.

These two lines will get the indices of your data, as you'd expect.

int index1 = data.indexOf(x);
int index2 = data.indexOf(y);

But all this does is ensure that your indices are valid, meaning the data were found. (Although the second check should also be -1 because the #indexOf method always returns the index, if found, or -1. Never -2.)

if (index1 == -1 || index2 == -1){ // changed to -1 on both
    return;
}

To actually do the swap, you need something like at the end of the other code in your swap method:

data.set(index1, y);
data.set(index2, x);

The #set method changes the value at the first argument index to the value in the second argument, so doing it twice will effectively swap the data.