java - Requiring user to enter only 1 char for a letter grade for a gpa program -

i trying finish java assignment , stuck on last bit. need make when user enters letter grade in order calculate g.p.a. can enter 1 letter.

for example, need receive error if enter aaa instead of a.

i stuck on how go doing this. works except 1 thing. new java great.

here class:

public class gpa {  private int sumcredits; private int sumpoints;     public int getpointsforgrade(char letter) {     int gradepoints;    switch (letter)    {    case 'a':    case 'a':       gradepoints = 4;       break;    case 'b':    case 'b':       gradepoints = 3;       break;    case 'c':    case 'c':       gradepoints = 2;       break;    case 'd':    case 'd':       gradepoints = 1;       break;    case 'f':    case 'f':       gradepoints = 0;       break;     default:       gradepoints = -1;       break;      }    return gradepoints;      }     public void constructor(){       sumcredits = 0;      sumpoints = 0;       return;   }    public void addgrade( char letter, int credits){        sumcredits = sumcredits + credits;      sumpoints = sumpoints + getpointsforgrade(letter) * credits;   }    public double calcgpa(){      double gpa;      if (sumcredits == 0){       return 0;       }      gpa = (double)sumpoints / sumcredits;      return gpa;   }    public int totalcredits(){     return sumcredits;    }    public int totalpoints(){     return sumpoints;    }  } 

and here test class, or gpa calculator

import java.util.*; import java.util.scanner;  public class computegpa {     public static void main (string [] args){     scanner scan = new scanner(system.in);    gpa gpa = new gpa();    int courses;    int credits;    char letter;         while (true){          system.out.println(" enter number of courses: ");          courses = scan.nextint();          if (courses > 0){              break;           }          else {             system.out.println(" invalid number of courses - must greater 0");                 }       }         (int = 1; i<= courses; i++)        {            while (true){              system.out.println(" enter grade ( 1 character )");             string grade = scan.next();             letter = grade.touppercase().charat(0);             if (letter == 'a' || letter == 'b' || letter == 'c' || letter =='d' || letter == 'f'  ){                break;             }             else {                system.out.println(" must enter a,b,c,d,f (upper or lower case)");             }              if (letter != 'a' || letter != 'b' || letter !=  'c' || letter != 'd' || letter != 'f'){                system.out.println("must 1 letter only");             }           }           while (true){             system.out.println(" enter credits: ");             credits = scan.nextint();              if (credits < 0 || credits > 9){                 system.out.println("invalid credits = must between 0 , 9, inclusively");             }             else {                break;             }          }             gpa.addgrade(letter, credits);          system.out.println(" points: " + gpa.getpointsforgrade(letter) );          system.out.println(" sum points: " + gpa.totalpoints() + " sum credits: " + gpa.totalcredits());         }        system.out.printf(" gpa: %.2f%n", gpa.calcgpa());      }  } 

you check see if there more entries in input. did not run code, simple way check.

 system.out.println(" enter grade ( 1 character )");             string grade = scan.next();             letter = grade.touppercase().charat(0);              if(grade.length() > 1){            // check size of input                system.out.println("you entered many letters");             }              else if (letter == 'a' || letter == 'b' || letter == 'c' || letter =='d' || letter == 'f'  ){                      break;                  }                  else {                      system.out.println(" must enter a,b,c,d,f (upper or lower case)");                  }