How to compare a list of integers and then sort it ascending?

I am learning how to sort and compare the dimension of squares in ArrayList. The dimension of the squares first ordered by length. If two squares have the same length, the squares requiring the shorter width should come first.

public class Square {
private int length;
private int width;
public Square(int length, int width) {
this.length = length;
this.width = width;
}
}
public class Dimension {
private int length;
private int width;
ArrayList<Square> Dimension = new ArrayList<>();
public void addSquare(int length, int width) {
Dimension.add(new Square(length, width));
}
public void sortDimension() {
Collections.sort(Dimension, (length, width) -> length.compareTo(width));
}
public static void main(String[] args) {
Square square;
Dimension dimension = new Dimension();
dimension.addSquare(10, 5);
dimension.addSquare(8, 8);
dimension.addSquare(10, 2);
dimension.addSquare(12, 10);
dimension.addSquare(8, 5);
dimension.sortDimension();
for(int i=0; i<Dimension.size();i++ ) {
System.out.println(Dimension.get(i));
}
}
}

I found the Collections.sort(List, (obj1, obj2) -> obj1.compareTo(obj2)) on the internet, but it does not work on my code. The compiler says cannot convert from Obj to int.

I need your suggestion on this matter. Thank you.

1. Don't use the word Dimension everywhere, for the class, for naming variables, here are suggestions

   ArrayList<Square> dims = new ArrayList<>(); // for the class attribut
   Dimension value = new Dimension(); // in the main
   

2. When providind a custom Comparator, you get a pair of object and need to determine the first one, you don't get it's properties

   Collections.sort(dims, (o1, o2) -> ); // Both o1 and o2 are Square instances
   // You'd get for something like
   Collections.sort(dims, (o1, o2) -> o1.getLength() == o2.getLength() ?
   o1.getWidth() - o2.getWidth() :
   o1.getLength() - o2.getLength());
   

3. But you can use Comparator interface that provides nice methods

   dims.sort(Comparator.comparing(Square::getLength).thenComparing(Square::getWidth));
   

Then in the main to access the list you need to go from the instance, as value.dims.size(). And with a nice toString in Square class it' be ok

class Square {
    private int length, width;
    public Square(int length, int width) {
        this.length = length;
        this.width = width;
    }
    public int getLength() {return length;}
    public int getWidth() {return width; }
    @Override
    public String toString() {return "Square{" + "length=" + length + ", width=" + width + '}';}
}

class Dimension {

    private ArrayList<Square> dims = new ArrayList<>();

    public static void main(String[] args) {
        Dimension value = new Dimension();
        value.addSquare(10, 5);
        value.addSquare(8, 8);
        value.addSquare(10, 2);
        value.addSquare(12, 10);
        value.addSquare(8, 5);
        value.sortDimension();

        for (int i = 0; i < value.dims.size(); i++) {
            System.out.println(value.dims.get(i));
        }
    }

    public void addSquare(int length, int width) {
        dims.add(new Square(length, width));
    }
    public void sortDimension() {
        dims.sort(Comparator.comparing(Square::getLength).thenComparing(Square::getWidth));
    }
}