How to have Kotlin "Listen" when a function finish executing Successfully - kotlin

Question

This is my first time using Kotlin, I have to write a simple command-line application where it takes a list of user input strings. Valid inputs are only "Apple" or "Orange" and calculate the price (which is 60 cents and 25 cents respectively). I'm having some trouble with the 3rd requirement
"Build a service that listens for when orders are complete and sends a notification to the customer regarding its status and estimated delivery time. The Mail service subscribes to events from the Orders service and publishes the appropriate event that the customer (you) is able to read from the terminal"
this is what I have done so far
MainApp.tk
import java.util.Scanner
import kotlin.system.exitProcess;
import app.Checkout;
var shopRunning = true;
var applecount = 0;
fun main(args: Array<String>) {
while (shopRunning) {
println("Welcome to Express Store");
println("1. Checkout");
println("2. exit");
var userOption = 0;
//request the user to eneter an option
//if user eneter a options that is not valid it will keep looping til option that is enterd is accepted;
var userSeletedOption = false;
val inputScanner = Scanner(System.`in`);
while (!userSeletedOption) {
print("Select an Option: ");
userOption = inputScanner.nextInt();
//if input entered by the user is not accepted and invaliud message is printed and is promted to enter an option again.
if (userOption != 1 && userOption != 2) {
println("Invalid input detected!");
} else {
userSeletedOption = true;
}
}
if (userOption == 1) {
val checkout = Checkout();
println("We currently have apples and oranges in Stock.")
var list: MutableList<String> = ArrayList();
println(list.size);
var doneAddingToCart = false;
while(!doneAddingToCart){
print("enter name of item to be enter or exit to finish adding to the cart: ")
var item = inputScanner.next();
if(item.equals("exit")){
doneAddingToCart=true;
}
else{
list.add(item);
}
}
if(checkout.verify(list)){ //checks if list has any item that is not an apple or orange
println("Thank you for your Pruchse");
val cost = checkout.Chasher(list)
println("You bought: "+ list.toString());
print("your total is: "+ cost);//returns the total cost
exitProcess(1);//exits from the application
}
} else if (userOption == 2) {
print("Have a great day.");
exitProcess(1);
}
}
}
CheckOut.tk
class Checkout {
//checks if the user entered any invaild items
public fun verify (cart: MutableList<String>) : Boolean{
for(item in cart){
if(!item.equals("Apple") && !item.equals("Orange")){
return false;
}
}
return true;
}
public fun Chasher (cart: MutableList<String>) : Double{
var total = 0.0;
var orangecount = 0;//step 2 offers
var applecount = 0;//step 2 offers
for(item in cart){//step 1 function
if(item.equals("Apple") || item.equals("apple")){
applecount+=1;
total= total + 0.6;
}
if(item.equals("Orange") || item.equals("orange")){
orangecount +=1;
total=total +0.25;
}
}
if(orangecount ==3){//buy three for the price of 2.step 2
println("You qaulidified for our buy 3 oragnes for the price of 2 offer")
total -=0.25;
}
if(applecount ==1){//buy one aple get 1 free. step 2
println("You buy 1 apple get one free")
cart.add("Apple");
}
return total;
}
}
I don't need to send an email just send a message to the command line. Currently, I'm just printing messages (just to see if what I currently have even works). Yeah, I know there many spelling errors, english and writing was never my strongest subject

Answer 1

I can only provide three hints that might help you:
If you exit your program using System.exit, use 0 if the run did not have any problem. (Excerpt from JavaDoc: "The argument serves as a status code; by convention, a nonzero status code indicates abnormal termination.")
For checking equality, simply use == which corresponds to equals in Java. In your special case however, you can use item.equals("apple", ignoreCase=true) or simply item.equals("apple", true).
I'm not sure what the author of your task exactly expects as a solution.
In can imagine you are expected to use lambdas.
An example: Your could refactor your Checkout class like that:
class Checkout {
/**
* Checks if the given [cart] contains only apples and oranges,
* and calls [onSuccess].
* If also other articles are contained, [onSuccess] is not called.
*/
fun verify(cart: List<String>, onSuccess: (List<String>) -> Unit): Unit {
for (item in cart) {
if (!item.equals("apple", true) && !item.equals("Orange", true)) {
return
}
}
onSuccess(cart)
}
}
And then call
val cart = listOf("Orange", "Apple", "apple", "orange")
Checkout().verify(cart, { cart: List<String> ->
println("Thanks you for your purchase: $cart")
})
or even shorter (curly brackets are outside of parenthesis)
Checkout().verify(cart) { cart: List<String> ->
println("Thanks you for your purchase: $cart")
}
What I did here was to extract what is executed if your validation succeeds:
For that, I used a lambda function that accepts a list of articles/strings (List<String>) and returns something I ignore/don't care about -> Unit.
The advantage of that approach is that callers of your verify method can decide what to do on success at their liking because they can pass a lambda function around like any other variable. Here:
val cart = listOf("Orange", "Apple", "apple", "orange")
val onSuccess = { cart: List<String> ->
println("Thanks you for your purchase: $cart")
}
Checkout().verify(cart, onSuccess)
You could also extend Checkout to allow an observer to register.
I deliberately kept the code very simple. Normally you would allow multiple observers to register, only expose what clients are supposed to see and hide the rest, etc.
class Checkout(
val onSuccess : (List<String>) -> Unit
) {
fun verify(cart: List<String>): Unit {
for (item in cart) {
if (!item.equals("apple", true) && !item.equals("Orange", true)) {
return
}
}
onSuccess(cart)
}
}
val checkout = Checkout({ cart: List<String> ->
println("Thanks you for your purchase: $cart")
})
and then
val cart = listOf("Orange", "Apple", "apple", "orange")
checkout.verify(cart)
Be sure to check out https://play.kotlinlang.org/byExample/04_functional/01_Higher-Order%20Functions to learn more about lambda / higher-order functions.