A bitcoins convert to teach how to use Kotlin

The master has the final project running. It is a command line dollar to bitcoin converter. The intention is to follow the steps from 1 up to 9 (each step is git tag) and complete the challenges described in the TODO's. Each step is described below

The step 1 shows how to configure a Kotlin project using Gradle, the challenges in step are:

• Apply the Kotlin plugin
• Add Kotlin build script dependency
• Add the Kotlin standard library

On step 2 we print our first hello world

• Create the main function
• Print on stdout the Hello World message

In step 3 the first class should be created, and the tests are introduced, from now all steps should have the unit tests passing

• Create a class property
• Uses @SerializedName to read the json value
• Make the tests pass

Create the first function in a way that makes the tests pass

• Create a function named "execute" that returns a list of Tickers
• Inside the execute function uses the dataSource.getTickets using "bitcoin" as coin
• The previous method returns a retrofit object, you need to call their execute() method and then call the body() method
• At this point you have a nullable list, uses the safe call "?." to filter the list to have only bitcoin tickers: ?.filter { it.id == "bitcoin" }
• Uses the elvis operator to send an emptyList if you receive a null: ?: emptyList()

We see the data class powers in action.

• Create a data class named DollarBitcoinModel
• Add two string values to DollarBitcoinModel data class named: dollar and bitcoin
• Make the tests pass

lets extend it

• Create a extension function to Double class named toBitcoin()
• Use the pattern "0.00000000 BTC" to format the double value and return a string
• Make the tests pass

The step 7 you will use the previous classes to fill a function that converts the dollar to bitcoin

• Go to DollarBitcoinConverter class and inside the method execute do the follow steps:
• Using the safe operator "?." call to toDoubleOrNull to get the double value from the dollar argument and save on a value named amount
• Using the smart cast check if the amount is a non null double: "if (amount is Double)"
• If the amount is null throw an IllegalArgumentException with the message "Cannot convert $dollar"
• If the amount is a valid non null double, call "getBitcoinTicker.execute()" to get a list of tickers to make the conversion
• Filter the ticker list to remove all null tickers and all tickers with null price: ".mapNotNull { it.price?.toDoubleOrNull() }"
• Filter again to remove values less than zero: .filter { it > 0.0 }
• Map the list to have the converted value: .map { amount / it }
• Map the list with converted values to our data class model: .map { DollarBitcoinModel(amount.toDollar(), it.toBitcoin()) }
• Get the first element of the list or null if after the filters we have no more elements: .firstOrNull()
• Uses the elvis operator "?:" to throw an NoSuchElementException with the message "No available ticker" if the pipe result is null
• Make the tests pass

Putting all together

• Go to Main.kt file and inside the main function do the follow:
• Print the question: "How much dollars do you want to convert?"
• Read the user input with "readLine()" and save it on a value
• Print the message "Please wait meanwhile I convert it"
• Create a DollarBitcoinConverter instance
• Call the "execute" function sending the user input and save the return into a value named model
• Print the follow message: "${model.dollar} dollars is equal to ${model.bitcoin}"
• Put the "execute" call inside a try/catch and if there is an exception print: "Error ${e.message}"

Fork the project and try to implement some of the following features:

• Bitcoin to dollar converter (the reverse hand)
• Convert to another cryptocurrency (Dashcoin, Ether, Dogecoin, Litecoin, etc…)
• Convert to other currencies (Pesos, Euro, Yen, Real, etc…)
• Add GUI (Graphical User Interface), it can be done with an Android application, or a website, or even the native Java GUI library.