Merge Sort Implementation java

I'm trying to implement my own Mergesort function but am having a hard time figuring out what's not working.

The output I get for UnSorted: [6, 1, 2, 7, 2, 3, 9, 7, 6] is Sorted: [2, 3, 6, 1, 2, 7]

Here's what I have so far:

public class mergeSort {

    public static void main(String[] args) {
        List<Integer> l = new ArrayList<Integer>();
        Random rd = new Random();

        for (int i = 1; i < 10; i++) {
            l.add(rd.nextInt(10) + 1);
        }   

        System.out.println("UnSorted: " + l);
        msort(l);
        System.out.println("Sorted: "+msort(l));
    }

    public static List<Integer> msort(List<Integer> l) {     
        if (l.size() <= 1) {
            return l;
        }

        List<Integer> left = new ArrayList<Integer>();
        List<Integer> right = new ArrayList<Integer>();

        for (int i = 0; i < (l.size() / 2); i++) {
            left.add(l.get(i));
        }
        for (int i = l.size() / 2; i < l.size(); i++) {
            right.add(l.get(i));
        }

        msort(left);
        msort(right);
        //System.out.println(left + "" +right);

        return join(left,right);
    }

    public static List<Integer> join(List<Integer> left, List<Integer> right) { 
        /*if (right.size() == 0) {
            return left;
        }
        if (left.size() == 0) {
            return right;
        }*/

        List<Integer> fin = new ArrayList<Integer>();
        // pointers
        int lp = 0, rp = 0, fp = 0;

        while (lp < left.size() && rp < right.size()) { 
            if (left.get(lp) < right.get(rp)) {
                fin.add(left.get(lp));  
                lp++;
            } else {
                fin.add(right.get(rp));  
                rp++;        
            }
            fp++;
        }   
        return fin;
    }       
}

Solution

There are couple of problems with your code. Your approach is right though

In join method you are leaving some of the elements in list because of your while loop is using

lp < left.size() && rp < right.size()

which will loop until one of the lists have been added to the fin and there might still be some element left in other list. so you need two more loops to accommodate this:
```
while(lp < left.size()) {
    fin.add(left.get(lp++));
}
while(rp < right.size()) {

    fin.add(right.get(rp++));
}
```
there is problem in your msort method as well you are not using the return values from msort so you need this:

left = msort(left);

right = msort(right);

Hope this helps.