How to test a class with private methods, fields, and inner classes in Java?

Hi everyone! I'm a beginner Java developer and I'm having a problem writing unit tests. In my project, there is a class `OrderProcessor` that contains private methods, fields, and even an inner class. When I try to write tests for it using JUnit, I cannot access these elements. Here's a simplified example of my class: ```java public class OrderProcessor { private List orders = new ArrayList<>(); private static final double TAX_RATE = 0.2; public double processOrder(Order order) { validateOrder(order); double total = calculateTotal(order); orders.add(order); return total; } private void validateOrder(Order order) { if (order == null || order.getItems().isEmpty()) {