This is an exercise to refactor and add test cases to a 'PaymentService' demo.

The steps for making a payment are:

1. Lookup the account the payment is being made from.
2. Check that the account is in a valid state to make the payment.
3. Deduct the payment amount from the account’s balance and update the account in the database.

Below are the common symptoms of God Object:

• SOLID Principle is not being followed and advocates for Anti-Pattern.
• Tight coupled functionalities will be difficult to maintain and enhance.
• Unit test writing will be very difficult.
• No Divide and Conquer strategy.
• Keeping all complexities in one place.

Original Class diagram:

God object can be refactored and fixed. Below are the ways.

• Adherence to SOLID principals

  Use "Interface Segregation" principle and delegate calls to the new extracted classes. "Open to extension, closed to modification" principle, extract out each of the payments types to a separate classes and abstract processing functionality into dedicated payment types.

• Testability

  Write unit test for each method.

• Readability

  Clear and Elegant code.

Revised Class diagram: