How to convert and cast an object list to an integer list?

May 7, 2022

Bellow code snippet errors with:

 "Type mismatch: cannot convert from element type Object to Integer".

public static List<Integer> quickSort(List<Integer> arr) {
    if(arr.size() == 1) {
        return arr;
    }
    int mid = arr.size() / 2;
    List left =  Arrays.asList(arr).subList(0, mid);
    List right =  Arrays.asList(arr).subList(mid, arr.size());
    Collections.sort(left);
    Collections.sort(right);
    for (Integer x : left) {
        right.add(x);
    }
    return arr;
}

>Solution :

There’s a logic bug, Arrays.asList(arr) actually creates a list of lists List<List<Integer>> while it looks like what you want is a List where arr already is one..

Also you need to properly parameterize List left and List right as List<Integer> otherwise it’s treated as default type Object.

public static List<Integer> quickSort(List<Integer> arr) {
    if(arr.size() == 1) {
        return arr;
    }
    int mid = arr.size() / 2;
    List<Integer> left =  arr.subList(0, mid);
    List<Integer> right =  arr.subList(mid, arr.size());
    Collections.sort(left);
    Collections.sort(right);
    for (Integer x : left) {
        right.add(x);
    }
    return arr;
}

EDIT: also returning arr in the return statement means your function does nothing as it just spits out the unedited input.