Separating Interface from Logic - Java - user-interface

Question

I'm quite new to programming and I'd like help/guidance on this concept please - "separating the user interface from logic". I'm assigned a simple task of designing a currency converter (it's pretty much a very basic code as it asks for the conversion rate - but that's the given spec) and below is the code I've used:
public class CurrencyConverter
{
public static void main(String[] args)
{
System.out.println("Welcome to Currency converter!" + "\n");
System.out.println("Please select an option below: ");
System.out.println("1 >> Pound to Euro " +
"\n2 >> Euro to Pound");
Scanner s = new Scanner(System.in);
String selection = s.next();
switch (selection)
{
case "1":
System.out.println("Enter the conversion rate from Pound to Euro");
break;
case "2":
System.out.println("Enter the conversion rate from Euro to Pound");
break;
}
Scanner rate = new Scanner(System.in);
double conversionRate = rate.nextDouble();
System.out.println("Now enter the amount to be converted: ");
Scanner input = new Scanner(System.in);
double amount = input.nextDouble();
double totalValue = conversionRate * amount;
System.out.println(totalValue);
}
There's a specific instruction to not do it this way (i.e. just one class) and also to not use GUI. Two classes are to be used, one for the user interface and the other for the conversion (logic). I've tried putting the code below in a second class "Conversion" but it doesn't work.
double totalValue = conversionRate * amount;
Any tip/help would be much appreciated! By the way, if you know how to do it using GUI, that'd be helpful too. But of course my main problem is the one above.
Thanks.

Answer 1

Separating ui from logic allows you to increase scalability and readability of your code. In your case one class should provide methods to support interaction with user in order to provide necessary data and other that will handle convertion process. Both classes should not be aware of each other.
For example ui can be implemented as follows:
public class ConverterUI {
public Double askForConversionRate() {
System.out.println("Welcome to Currency converter!" + "\n");
System.out.println("Please select an option below: ");
System.out.println("1 >> Pound to Euro "
+ "\n2 >> Euro to Pound");
Scanner s = new Scanner(System.in);
String selection = s.next();
switch (selection) {
case "1":
System.out.println("Enter the conversion rate from Pound to Euro");
break;
case "2":
System.out.println("Enter the conversion rate from Euro to Pound");
break;
}
Scanner rate = new Scanner(System.in);
return rate.nextDouble();
}
public Double askForAmountToConvert() {
System.out.println("Now enter the amount to be converted: ");
Scanner input = new Scanner(System.in);
return input.nextDouble();
}
public void showResult(Double result) {
System.out.println("Convertion result is: " + result);
}
}
It has three simple methods that you can use to get data from user but there is no information about what to do with it. Converter class is responsible for this part:
public class Converter {
private Double conversionRate;
private Double amount;
public Double convert() {
return conversionRate * amount;
}
public void setConversionRate(Double conversionRate) {
this.conversionRate = conversionRate;
}
public void setAmount(Double amount) {
this.amount = amount;
}
}
It allows you to set convertion rate and amount values and make some calculations with convert() method. It doesn't need to know what is the source of data - you should support valid arguments and it will provide response.
In this situation main class can look like this:
public class Main {
public static void main(String[] args) {
ConverterUI ui = new ConverterUI();
Converter converter = new Converter();
Double convertionRate = ui.askForConversionRate();
converter.setConversionRate(convertionRate);
Double amount = ui.askForAmountToConvert();
converter.setAmount(amount);
Double result = converter.convert();
ui.showResult(result);
}
}
We create two objects - ui and converter. Calling ui methods provides data which are set to converter, and result of convertion is passed back to ui to inform user.
This approach allows you to add new UI (defining an interfaces would be a nice idea) and converter implementations without need of editing existing ones.

Answer 2

If you make another class called Conversion, it will not have access to the conversionRate and amount variables, since those variables only exist within the main method of the CurrencyConverter object.
You would have to create an object of the Conversion class and pass conversionRate and amount as method parameters. You might think about passing conversionRate to the constructor so that you could use the same rate to convert many different amounts, only passing the amount each time:
// Construct an object of the Conversion class with
// conversionRate as a parameter.
Conversion c = new Conversion(conversionRate);
// Call the convert method of the Conversion object using amount as a param.
System.out.println("Value of " + amountOne +
" Pounds in Euros: " + c.convert(amountOne));
System.out.println("Value of " + amountTwo +
" Pounds in Euros: " + c.convert(amountTwo));
You could later just assign c to a new Conversion object if you wanted to change the rate:
c = new Conversion(aDifferentRate);
The same goes for your Scanner objects. You can reuse the same variable over and over again. In your example, I don't think you even need to assign it to a new object, but you could:
Scanner in = new Scanner(System.in);
String str = in.next();
in = new Scanner(3.14159);
double d = in.nextDouble();