为什么数组值在切换情况下不保留？

Question

Why aren't the array values saving in my switch functions? First I pressed 1, created my roster values. After I created the values, I press 2 it leads to ArrayOutOfBound error. Can anyone tell me what am I doing wrong?

Please ignore the FileNotFoundExceptions. Need to work on that part after I fix what I'm doing wrong.

public static Scanner keyboard = new Scanner(System.in);
public static int numStudents;
public static String[] nameOfStudent;
public static int[] broncoID;
public static int[] grade;

^ These are my declared variables

public static void main(String[] args) throws FileNotFoundException {

    startMenus(keyboard);
}

public static void startMenus(Scanner sc) throws FileNotFoundException {
    while (true) {
        System.out.println("(Enter option # and press ENTER)\n");
        System.out.println("1. Create new Roster");
        System.out.println("2. Display Current Roster");

        int option = sc.nextInt();

        sc.nextLine();

        switch (option) {
        case 1:
            createRoster();
            break;
        case 2:
            displayRoster();
            break;
        default:
            System.out.println("Unrecognized Option!\n");
        }
    }
}

public static void createRoster() throws FileNotFoundException {

    System.out.println("Please list how many students there are.");

    numStudents = keyboard.nextInt();

    nameOfStudent = new String[numStudents];
    broncoID = new int[numStudents];
    grade = new int[numStudents];

    keyboard.nextLine();

    for(int i = 0; i < numStudents; i++){
        System.out.println("Please enter full name of Student.");
        nameOfStudent[i] = keyboard.nextLine();
        System.out.println("Now please enter the BroncoID of Student.");
        broncoID[i] = keyboard.nextInt();
        System.out.println("Now please enter grade of Student (0-100)");
        grade[i] = keyboard.nextInt();
        keyboard.nextLine();
    }
    System.out.println("Roster successfully created!");
    for (int i = 0; i < numStudents ; i++){
        System.out.printf("Name: %9s\t Bronco ID: %5d \t Grade: %2d \n", nameOfStudent[i], broncoID[i], grade[i]);
    }
}

public static void displayRoster(){
    System.out.printf("Name: %9s\t Bronco ID: %5d \t Grade: %2d \n", nameOfStudent[1], broncoID[1], grade[1]);

}

The printf in the first case works, but it doesn't work in the second case. Error I get:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1
at Roster.displayRoster(Roster.java:115)
at Roster.startMenus(Roster.java:36)
at Roster.main(Roster.java:16)

Answer 1

An Array always starts with the index 0. You added one "Roster" to your Array. This Roster will get index 0. In your displayRoster() Method you want to show index 1, but your array only has index 0. So it will get the ArrayIndexOutOfBoundsException.

Instead of:

System.out.printf("Name: %9s\t Bronco ID: %5d \t Grade: %2d \n", nameOfStudent[1], broncoID[1], grade[1]);

Try:

System.out.printf("Name: %9s\t Bronco ID: %5d \t Grade: %2d \n", nameOfStudent[0], broncoID[0], grade[0]);

Answer 2

you are using 2 cases in switch statement:

case 1 and 2

However, you're taking input as 1 for createroster and 4 for displayroster

either use "case 1" and "case 4" in switch statement. or use:

System.out.println("1. Create new Roster");
System.out.println("2. Display Current Roster");

Answer 3

1. Better modify the method displayRoster as :

    public static void displayRoster() { System.out.println("Enter the record number to be displayed[1 to numStudents :"); int i = keyboard.nextInt(); if(i <= nameOfStudent.length) System.out.printf("Name: %9s\\t Bronco ID: %5d \\t Grade: %2d \\n", nameOfStudent[i-1], broncoID[i-1], grade[i-1]); else System.out.println("This is where you are trying to access element out of bound."); } 

2. Input option = 4 directs to the default switch case in your code which should S.out " Unrecognized Option!"