title: How to improve/edit in this UML class diagram?
body: <p>How can I improve this UML class diagram and what mistakes I made?</p>
  
  <p>I'm really confused about the relationship between Expense and Category, because Category and Expense can exist by themselves: you can register Expense and not assign it to a category, or do the opposite, you can add new Categories to the categories list, and so on...</p>
  
  <p>Do I really need separate List classes?</p>
  
  <p><a href="https://i.stack.imgur.com/AP1pk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/AP1pk.png" alt="enter image description here"></a></p>