To design a test suite for a banking application, I would follow a layered approach:

Unit Tests (20-30% of total tests)

• Focus on individual components, such as calculators, validators, and data access objects
• Use mocking libraries to isolate dependencies
• Test specific scenarios, edge cases, and error handling
• Examples: testing interest rate calculations, password hashing, and data encryption

Integration Tests (40-50% of total tests)

• Focus on interactions between components, such as API calls, database interactions, and file system access
• Use test doubles and stubs to simulate dependencies
• Test workflows, such as user authentication, transaction processing, and account updates
• Examples: testing login functionality, transaction processing, and account balance updates

End-to-End Tests (30-40% of total tests)

• Focus on user interactions, such as UI workflows, user journeys, and business processes
• Use tools like Selenium, Cypress, or Appium to simulate user interactions
• Test critical user flows, such as depositing funds, transferring money, and paying bills
• Examples: testing online banking login, fund transfer, and bill payment workflows

This layered approach ensures comprehensive testing of the banking application, covering individual components, interactions between components, and user interactions.