Issue trying to create a Bubble Sort using ArrayList<Integer>

Question

Hi I'm trying to figure out how to use BubbleSort in Java and my code is erroring and I don't know why

import java.util.ArrayList;

public class SortsRunner {

    public static void BubbleSort(ArrayList<Integer> nums) {
        ArrayList<Integer> arr = new ArrayList<Integer>();
        int n = arr.size();
        for (int i = 0; i < n-1; i++) 
            for (int j = 0; j < n-i-1; j++) 
                if (arr.get(j) > arr.get(j+1)) 
                { 

                    int temp = arr.get(j); 
                    arr.get(j) = arr.get(j+1); 
                    arr.get(j+1) = temp; 
                }
        }
    public static void SelectionSort(ArrayList<Integer> nums) {

        }
    public static void printArrayList(ArrayList<Integer> nums) {
        for(int i = 0; i < nums.size(); i++) {
            System.out.println(nums.get(i) + " ");
            }
        System.out.println();
        }
    public static ArrayList<Integer> makeRandomArrayList() {
        ArrayList<Integer> nums = new ArrayList<>();
        for(int i = 0; i < (int)(Math.random() * 11) + 5; i++) {
            nums.add((int)(Math.random() * 100));
            }
        return nums;
        }

public static void main(String[] args) {
printArrayList(makeRandomArrayList());

}

}

When I get to arr.get(j) = arr.get(j+1); and arr.get(j+1) = temp; the left side errors saying "The left-hand side of an assignment must be a variable." can anyone help me fix this?

Answer 1

arr.get(j) = arr.get(j+1);
arr.get(j+1) = temp; 

You're trying to assign a value to the result of a method call.

You just can't do this. You can only assign to a local variable, a field in the current class, a field access (eg foo.bar = ... ) or an array element (eg foo[0] = ... ).

Instead, you should use set to update a list element:

arr.set(j, arr.get(j+1));
arr.set(j+1, temp);

---

For the specific case of swapping two elements around in a list, you can instead use Collections.swap :

Collections.swap(arr, j, j+1);

Answer 2

You are doing several things wrong.

1. The obivous get and set issues already mentioned.
2. The fact that your are sorting an empty list . You pass in nums but sort the one you create which is empty.
3. You should use a boolean to prevent unnecessary repeats of the outer loop . Think of it like this, if you don't make a swap on the first iteration of the outer loop, then you won't swap on subsequent iterations.

And one style suggestion. Don't use loops or if statements without {} . Even if they only contain a single line of code. You will be less likely to make coding errors if you do so.

Try the following:

    public static void BubbleSort(List<Integer> nums) {
        int n = nums.size();
        for (int i = 0; i < n; i++) {
            boolean swapped = false;
            for (int j = 0; j < n-1; j++) {
                if (nums.get(j) > nums.get(j + 1)) {
                    int temp = nums.get(j);
                    nums.set(j, nums.get(j + 1));
                    nums.set(j + 1, temp);
                    swapped = true;
                }
            }
            if (!swapped) {
                break;
            }
        }
    }