Java货币转换器输入验证问题

Question

I'm supposed to add a while loop to a currency converter, and it's supposed to check if the user enters any letter other than Y, y, P, or p, and prompt them to try again re-enter their currency type.

I'm struggling to know where to place it in my code. Any help would be greatly appreciated. The code for the currency converter is:

import java.util.Scanner;

public class CurrencyConverter 
{

public static void main(String[] args) 
{

    //Store these 2 conversion rate as constant (final) variables
    final double PESO = 20.37;
    final double YEN = 114.37;

    double total =0;

    //Get the data from the user
    Scanner k = new Scanner(System.in);

    //Get the amount of USD
    System.out.println("how much money do you want to convert?");
    double usd = k.nextDouble();

    //Get the conversion type (peso or yen)
    System.out.println("do you want to convert to Peso or Yen?");
    char type = k.next().charAt(0); //get 1st char of user input

      switch(type)
      {
        case 'p':
        case 'P':
          //convert and print
          total = usd * PESO;
          System.out.printf("$%.2f = %.2f Peso\n", usd, total);
          break;
        case 'y':
        case 'Y':
          //convert and print
          total = usd * YEN;
          System.out.printf("$%.2f = %.2f Yen\n", usd, total);
          break;
        default:
          System.out.println("Sorry Invalid Currency type, " + 
                             "please try again");
          System.out.println("do you want to convert to Peso or Yen?");
          type = k.next().charAt(0);
      }

      if ((usd >= 1000) && (type=='p' || type=='P'))
      {
        System.out.println("You're going to have a blast in Mexico");
      }
      else if ((usd > 5000) && (type=='y' || type=='Y'))
      {
        System.out.println("Have a great time in Japan!");
      }
      else if (usd < 10)
      {
        System.out.println("Haha you're broke!");
      }     

    }

}

Answer 1

You just need to enclose the input and validation code in a while loop, and use a flag to control whether to loop back. Something along these lines:

boolean invalidInput;
do {
    System.out.println("do you want to convert to Peso or Yen?");
    char type = k.next().charAt(0); //get 1st char of user input

    invalidInput = false;
    switch(type)
    {
    case 'p':
    case 'P':
      //convert and print
      total = usd * PESO;
      System.out.printf("$%.2f = %.2f Peso\n", usd, total);
      break;
    case 'y':
    case 'Y':
      //convert and print
      total = usd * YEN;
      System.out.printf("$%.2f = %.2f Yen\n", usd, total);
      break;
    default:
      System.out.println("Sorry Invalid Currency type, " + 
                         "please try again");
      invalidInput = true;
    }
} while (invalidInput);

Answer 2

You can just wrap around the switch , then control it with break or continue

public static void main(String[] args) {

    //Store these 2 conversion rate as constant (final) variables
    final double PESO = 20.37;
    final double YEN = 114.37;

    double total = 0;

    //Get the data from the user
    Scanner k = new Scanner(System.in);

    //Get the amount of USD
    System.out.println("how much money do you want to convert?");
    double usd = k.nextDouble();
    char type = ' ';

    while(true) {
        //Get the conversion type (peso or yen)
        System.out.println("do you want to convert to Peso or Yen?");
        type = k.next().charAt(0); //get 1st char of user input

        switch(type) {
        case 'p':
        case 'P':
            //convert and print
            total = usd * PESO;
            System.out.printf("$%.2f = %.2f Peso\n", usd, total);
            break;
        case 'y':
        case 'Y':
            //convert and print
            total = usd * YEN;
            System.out.printf("$%.2f = %.2f Yen\n", usd, total);
            break;
        default:
            System.out.println("Sorry Invalid Currency type, " + "please try again");
            continue;
        }
        break;
    }

    if ((usd >= 1000) && (type=='p' || type=='P')) {
        System.out.println("You're going to have a blast in Mexico");
    } else if ((usd > 5000) && (type=='y' || type=='Y')) {
        System.out.println("Have a great time in Japan!");
    } else if (usd < 10) {
        System.out.println("Haha you're broke!");
    }

}

Answer 3

Create a bool for loop competetion set to false before the switch

bool selected = false;

Next, create a while loop around the switch, set to loop while the bool "selected" is false

  while(!selected){
      switch(type)

Then change the bool to true after a successful user selection

        case 'p':
        case 'P':

            selected = true;

Answer 4

Try this code, its very simple:

import java.util.Scanner;

public class HelloWorld 
{

public static void main(String[] args) 
{

    //Store these 2 conversion rate as constant (final) variables
    final double PESO = 20.37;
    final double YEN = 114.37;

    double total =0;

    //Get the data from the user
    Scanner k = new Scanner(System.in);

    //Get the amount of USD
    System.out.println("how much money do you want to convert?");
    double usd = k.nextDouble();

    //Get the conversion type (peso or yen)
    System.out.println("do you want to convert to Peso or Yen?");
    char type = k.next().charAt(0); //get 1st char of user input

    while( true  ) {
      switch(type)
      {
        case 'p':
        case 'P':
          //convert and print
          total = usd * PESO;
          System.out.printf("$%.2f = %.2f Peso\n", usd, total);
          break;
        case 'y':
        case 'Y':
          //convert and print
          total = usd * YEN;
          System.out.printf("$%.2f = %.2f Yen\n", usd, total);
          break;
        default:
          System.out.println("Sorry Invalid Currency type, " + 
                             "please try again");
          System.out.println("do you want to convert to Peso or Yen?");
          type = k.next().charAt(0);
          continue;
      }
      break;
    }

      if ((usd >= 1000) && (type=='p' || type=='P'))
      {
        System.out.println("You're going to have a blast in Mexico");
      }
      else if ((usd > 5000) && (type=='y' || type=='Y'))
      {
        System.out.println("Have a great time in Japan!");
      }
      else if (usd < 10)
      {
        System.out.println("Haha you're broke!");
      }     

    }

}

All I added is while loop with Boolean "true", a continue and break statement.