Question

Working on a project for school and I’m getting a error when I try to enter the number of students for the array. The error is

Exception in thread “main” java.lang.ArrayIndexOutOfBoundsException: -1
at Project1.enterStudents(Project1.java23)
at Project1.mainMenu(Project1.java59)
at Project1.enterStudents(Project1.java7)

The code I have written it below as always any help is appreciated.

import java.util.Scanner;

public class Project1{
public static void main(String[] args){
Scanner input = new Scanner(System.in);
Project1 project1 = new Project1();
project1.mainMenu();


}//main

int numOfStudents;
Student[] students = new Student[numOfStudents];

public void enterStudents(){
    Scanner input = new Scanner(System.in);
    System.out.println("Enter number of students");
    numOfStudents = input.nextInt();
    int i;
    for(i = 0; i <= numOfStudents - 1; i++){
        i--;
        System.out.println("Enter student's ID: ");
        students[i].getId();
        System.out.println("Enter student's first name: ");
        students[i].getFirst();
        System.out.println("Enter student's last name: ");
        students[i].getLast();
        System.out.println("Enter student's class: ");
        students[i].getStuClass();
    }


}

public void retrieveStuId(){
    Scanner input = new Scanner(System.in);
    System.out.println("Enter student id");


}

public void Exit(){
    System.exit(0);
}

public void mainMenu(){
    Scanner input = new Scanner(System.in);
    System.out.println("1 - Enter student info");
    System.out.println("2 - Retrieve student by ID");
    System.out.println("3 - Retrieve student by last name");
    System.out.println("4 - Update student");
    System.out.println("5 - Exit");
    int menuSelect = input.nextInt();

    if (menuSelect != 1 && menuSelect != 2 && menuSelect != 3 && menuSelect != 4 && menuSelect != 5)
        System.out.println("That is not a option");
    else
        switch (menuSelect){
            case 1: enterStudents();

            case 2: System.out.print("case 2");

            case 3: System.out.print("case 3");

            case 4: System.out.print("case 4");

            case 5: Exit();

        }
}

}//project1

class Student{
private int studentID;
private String firstName;
private String lastName;
private String stuClass;

public Student(){
}

public Student(int id, String first, String last, String c ){
    studentID = id;
    firstName = first;
    lastName = last;
    stuClass = c;
}

public void setID (int id){
    studentID = id;
}

public void  setStuClass (String c){
    stuClass = c;
}

public void setFirst(String first){
    firstName = first;
}

public void setLast(String last){
    lastName = last;
}

public String getFirst(){
    return firstName;
}

public String getLast(){
    return lastName;
}

public int getId(){
    return studentID;
}

public String getStuClass(){
    return stuClass;
} 

public String toString(){
    return "Student ID: " + studentID + " ---- " + "Student Name: " + firstName + "" + lastName + " ---- " + "Class:" + stuClass;
}


}

Answer 1

Look at this bit of code:

for(i = 0; i <= numOfStudents - 1; i++){
    i--;
    System.out.println("Enter student's ID: ");
    students[i].getId();

Now work out what the value of i is going to be on each line…

Why do you have the i--; line at all?

Note that this only addresses the first ArrayIndexOutOfBoundsException issue – once that’s fixed, you’ll end up with another ArrayIndexOutOfBoundsException because you’re initializing the array before asking for numOfStudents.

Once that’s addressed, you’ll get a NullPointerException because you’re trying to call methods via null references – you never actually create a new Student instance.

To be honest, this program is quite a long way from working – I’m not sure that Stack Overflow is going to provide the most effective teaching environment in this particular case. I would suggest you talk to your teacher and ask for some 1-on-1 tutoring.