Information hiding - have a private variable and public getter/setter. So you have a better control doing the right and having derived values.

public class Square {
    public int width;
    public int height;
    public int size;
}

mySquare.width = -1; 

public class Square {
    private int width;
    private int height;

    public int getWidth()  { return width;  }
    public int getHeight() { return height; }

    public void setWidth(int w) {
        if (w>=0) width = w;
    }
    public void setHeight(int h) {
        if (h>=0) height = h;
    }

    public int getSize() {
        return width * height;
    }
}