java - 如何让这段代码继续循环，直到输入正确的学号？

Question

如何让此代码继续询问“学生编号”，直到正确输入以字符“X”开头的学生编号，然后继续执行该程序？

import java.util.Scanner;
public class SDevCA2 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here

        //inputs
        //Option 1


        String userName;
        String passWord1;
        String passWord2;
        String eol;
        String userInput;
        int loopVal =0;
        char firstLetter = 'X';
        int userOption;
        final double FULLGRANT = 3000;
        final double GRANTSEVENTYFIVE = 2250;
        final double GRANTFIFTY = 1500;

        double softWareGrade =0;
        double mathsGrade =0;
        double systemGrade =0;
        double computerArchGrade =0;


        double gradeCount;
        final double SUBJECT_COUNT = 4;

        double averageGrade;
        int overallAverageResults;
        int feesOwedPercent;
        double feesOwedEuros;

        //Option 2

        double feesPaidByGrant =0;
        double feesNotPaidByGrant =0;
        int totalStudentsProcessed =0;
        int totalStudentsRecievedGrant =0;

        //Option 3

        int studentsWith100Paid =0;
        int studentsWith75Paid =0;
        int studentsWith50Paid =0;
        int studentsWithNoGrant =0;

        int categoryMostStudents;

        //End of Inputs
        boolean run = true;
        while ( run ) {
        Scanner in = new Scanner (System.in);

         //input of data
        //input of data

       System.out.println("Please enter your username: ");
       userName = in.nextLine();
       System.out.print("Please enter your password: ");
       passWord1  = in.nextLine();
       System.out.print("Please re-enter your password: ");
       passWord2  = in.nextLine();
       while (!passWord1.equals(passWord2))
        {

       System.out.println("Incorrect! Please try re-entering password ");
       System.out.print("Please enter your password: ");
       passWord1  = in.nextLine();
       System.out.print("Please re-enter your password: ");
       passWord2  = in.nextLine();

                    }


        //ABC Grant Menu.
        System.out.println("\n  --------------------------------");
       System.out.println ("Welcome to the ABC College Grant System  \n 1. Calculate Grant  \n 2. Fee Statistics   \n 3. Grant Category Information   \n 4. Exit");
       userOption = in.nextInt();


        //if Invalid Entry Error Message with the ABC Menu again



        //Option 1 
       switch ( userOption ) {
            case 1:


       String studentName;
       String studentNumber;         



       System.out.println ("Please Enter your Student Name: ");
       studentName = in.next();




       System.out.println ("Please Enter your Student Number: ");
       userInput = in.next();

       while ( userInput.charAt(0) == firstLetter) {
       System.out.println("Correct");
       break;

       }

       while ( userInput.charAt(0) != firstLetter)
       {


        System.out.println("No match, Student Numbr must begin with character capital X ");
       }   






       {





           //Error message if Student Number doesn't start with X

           System.out.println ("What was your Grade in Software Development");
           softWareGrade = in.nextDouble();

           System.out.println ("What was your Grade in Maths");
           mathsGrade = in.nextDouble ();

           System.out.println("What was your Grade in Systems Analasys");
           systemGrade = in.nextDouble ();

           System.out.println("What was your Grade in Computer Archetecture");
           computerArchGrade = in.nextDouble();


           //Formula

           gradeCount = softWareGrade + mathsGrade + systemGrade + computerArchGrade;
           averageGrade = gradeCount / SUBJECT_COUNT;

           //100% FEES PAID





           if (averageGrade >=80 && averageGrade <=100) {
               feesOwedPercent = 0;
               feesOwedEuros = 0;
               totalStudentsProcessed ++;
               totalStudentsRecievedGrant ++;
               studentsWith100Paid ++;
               feesPaidByGrant = feesPaidByGrant + FULLGRANT;
               System.out.println ("Student Name: " + studentName);
               System.out.println ("Student Number: " + userInput);
               System.out.println ("Average Grade: " +averageGrade);
               System.out.println("Fees Owed: " +feesOwedPercent);
               System.out.println("Fees not paid " + feesOwedEuros);

           }

           if  (averageGrade >=60 && averageGrade <80 ) {
               feesOwedPercent = 25;
               feesOwedEuros = 750;
               totalStudentsProcessed ++;
               totalStudentsRecievedGrant ++;
               studentsWith75Paid ++;
               feesPaidByGrant = feesPaidByGrant + GRANTSEVENTYFIVE;
               feesNotPaidByGrant = feesNotPaidByGrant + feesOwedEuros;
               System.out.println ("Student Name: " + studentName);
               System.out.println ("Student Number: " + userInput);
               System.out.println ("Average Grade" +averageGrade);
               System.out.println("Fees Owed: " +feesOwedPercent);
               System.out.println("Fees not paid " + feesOwedEuros );
           }

           if   (averageGrade >=40 && averageGrade <60) {
               feesOwedPercent = 50;
               feesOwedEuros = 1500;
               totalStudentsProcessed ++;
               totalStudentsRecievedGrant ++;
               studentsWith50Paid ++;
               feesPaidByGrant = feesPaidByGrant + GRANTFIFTY;
               feesNotPaidByGrant = feesNotPaidByGrant + feesOwedEuros;
               System.out.println ("Student Name: " + studentName);
               System.out.println ("Student Number: " + userInput);
               System.out.println ("Average Grade" +averageGrade);
               System.out.println("Fees Owed: " +feesOwedPercent);
               System.out.println("Fees not paid " + feesOwedEuros );


           }

           if ( averageGrade <40 ) {
               feesOwedPercent = 100;
               feesOwedEuros = 3000;
               totalStudentsProcessed ++;
               studentsWithNoGrant ++;
               feesNotPaidByGrant = feesNotPaidByGrant + feesOwedEuros;
               System.out.println ("Student Name: " + studentName);
               System.out.println ("Student Number: " + userInput);
               System.out.println ("Average Grade" +averageGrade);
               System.out.println("Fees Owed: " +feesOwedPercent);
               System.out.println("Fees not paid " + feesOwedEuros );
               break;

       }

                switch ( userOption) {
                    case 2:
                        System.out.println("The overall fess paid by the grant: €" +feesPaidByGrant );
                        System.out.println("The overall fees not paid by the grant: €" +feesNotPaidByGrant );
                        System.out.println("The total number of students processed is: " +totalStudentsProcessed );
                        System.out.println("The total number of students who recieved a grant is: " +totalStudentsRecievedGrant );
                        break;

    }
                switch ( userOption ) {
                    case 3:
                        System.out.print("How many stduents are given grants in the following catergory");
                        System.out.println("100% Paid: " +studentsWith100Paid );
                        System.out.println("75% Paid: " +studentsWith75Paid );
                        System.out.println("50% Paid: " +studentsWith50Paid );
                        System.out.println("No Grant Paid: " +studentsWithNoGrant );
                        break;
                }

                switch ( userOption) {
                    case 4: 
                        System.out.print("The number of stduents that are given grants in the following catergories: ");
                        System.out.println("\n");
                        System.out.println("100% Paid: " +studentsWith100Paid );
                        System.out.println("75% Paid: " +studentsWith75Paid );
                        System.out.println("50% Paid: " +studentsWith50Paid );
                        System.out.println("No Grant Paid: " +studentsWithNoGrant );

                        System.out.print("\n");
                        System.out.print("\n");
                        System.out.print("The catergory from which most grants are paid is: ");





                        run=false;
                        break;
                    default:
                }

    }
    }
    }
    }
}

Answer 1

提示用户输入学号然后接受输入的代码必须位于 while 循环内。尝试这样的事情:

boolean validNumber = false;
while (!validNumber) {
  System.out.println ("Please Enter your Student Number: ");
  userInput = in.next();

  if (userInput.charAt(0) == firstLetter) {
    validNumber = true;
  } else {
     System.out.println("No match, Student Numbr must begin with character capital X ");
  }
}