Execution error for Inheritance in Java

Question

My code will not execute from line 29 onwaard. I am beginner in java and do not know about exception handling and can not handle the NULL pointer exception thrown. Is there a way other than exception handling or there is flaw in my code?

public static void run(int a){
        Scanner scanf = new Scanner(System.in);
    switch (a) {
            case 1:
                int i=0;
                int q=0;
                while(i!=5 || q!=-99999){
                System.out.println("Enter Student Name");
                String x = scanf.nextLine();
                student[i].setName(x);
                System.out.println("Enter Registration Number");
                String z = scanf.nextLine();
                student[i].set_RegistrationNo(z);
                System.out.println("Enter -99999 to exit");
                i++;
                 q =scanf.nextInt();
                }
                break;
            case 2:
                System.out.println("Which Record do you want to change?\n 1.Name 2.Registration Number");
                int y=scanf.nextInt();
                if(y==1){
                    System.out.println("Enter Student Name");
                    student[y].setName(null);
                    String x = scanf.nextLine();
                    student[y].setName(x);
                }
                else if(y==2)
                {
                    student[y].set_RegistrationNo(null);
                    System.out.println("Enter Student Registration Number");
                    String z=scanf.nextLine();
                    student[y].set_RegistrationNo(z);
                }
                question();
                break;
            case 3:
                System.out.println("Enter record you want to delete");
                int k=scanf.nextInt();
                student[k].setName(null);
                student[k].set_RegistrationNo(null);
                question();
                break;
            case 4:
                System.out.println("Enter record you want to view/search");
                int c=scanf.nextInt();
                System.out.println("Name is");
                student[c].getName();
                System.out.println("Registration number is");
                student[c].get_registrationNo();
                question();
                break;
            case 5:
                for(int r=0;r<5;r++){
                    System.out.printf("Name of Student %d\n",r);
                    student[r].getName();
                    System.out.printf("Registration Number of Student %d\n", r);
                    student[r].get_registrationNo();
                } 
                question();
                break;
            case 6:
                break;
        }
    }
}