🟡 Medium: Currency Converter (Android Networking & Logic)

Challenge Overview

In this challenge, you are tasked with completing an Android application that performs live currency conversions. The MainActivity handles fetching exchange rates via Retrofit and updating the UI, but you must complete the core conversion mathematics.

Your goal is to implement the missing logic to ensure the currency conversions are accurate across multiple different currencies, supporting base exchange rates relative to USD.

Problem Description

You are given an Android project with a MainActivity.kt file. While the API fetching and spinner UI are already configured, the conversion shell is left for you.

The convertAmount function inside the companion object accepts the following parameters:

• amount: Double - The value to be converted.
• fromRate: Double - The exchange rate of the source currency relative to USD.
• toRate: Double - The exchange rate of the target currency relative to USD.

You need to perform the following operations:

1. Conversion Math: Implement the convertAmount function. It must calculate the target amount using the provided rates, taking into account that both rates are bound to a common base (USD).
2. Handle Edge Cases: Ensure that conversions cleanly handle decimal numbers, 0 amounts, and same-currency conversions natively without throwing unexpected errors.

Expected Output

The convertAmount function should simply return a Double:

• A Double representing the newly converted currency amount.

How to Run

Run the unit tests:

./gradlew test

The unit tests might fail initially or return a generic stub. Your task is to modify the convertAmount function inside MainActivity.kt until all tests in CurrencyConversionTest.kt and InputValidationTest.kt pass.

Important Implementation Notes

Formula Logic Think about how you convert between two currencies when both are given relative to USD. For example, if USD to INR is 83.0 and USD to EUR is 0.92, how do you convert EUR to INR?

Type Safety Keep all inputs and outputs as Double. Do not return NaN or unformatted strings. The UI bindings already handle decimal string formatting for 0.00 places.

Hints

• The mathematical formula should involve multiplying amount by a ratio of toRate and fromRate.
• Review your edge cases: If you convert 0.0 or translate to the exact same currency (e.g. 1.0 and 1.0), the result should seamlessly pass.
• Remember: The networking code in onCreate is already completed. Your structural problem solving and mathematical intuition are the keys here!