How to solve Cannot invoke "String.compareTo(String)" because "[]" is null?

Question

import java.util.Scanner;


public class Search {
    static Scanner scanner = new Scanner(System.in);
    static Scanner kb = new Scanner(System.in);
    static Scanner kb2 = new Scanner(System.in);
    public static void main (String[] args)
    {

        int choice;
        System.out.print("Choose a number of students: ");
        int n = scanner.nextInt();  
        String name[] = new String[n+1];
        String course[] = new String[n+1];
        int ID[] = new int[n+1];

        for(int i=1;i <= n; i++)
        {
            System.out.print("Enter ID number " + i + ": ");
            ID[i] = scanner.nextInt();
            System.out.print("Enter Student name " + i + ": ");
            name[i] = kb.nextLine();
            System.out.print("Enter Student course " + i + ": ");
            course[i] = kb2.nextLine();
            System.out.println("----------------------------------------");
        }

       
      
        do
        {
            choice = menu();
            if(choice == 1)
            {
                sortID(ID);
                printValues(ID);

            }else if(choice == 2)
            {
                nameSort(name,n);
                printName(name,n);
            }else if(choice == 3)
            {

            }
        }while(choice !=0);
    }

    public static int menu()
    {
        System.out.print("\n1. Sort by ID\n2. Sort by Name\n3. Search by ID\n4. Search by Name\n5. Search by Course\n6. Display Records In table Form.\nYour Choice: ");
        return scanner.nextInt();
    }

    public static void sortID(int []id)
    {
        int temp;
        int index, counter;
        for (counter=0; counter < id.length -1; counter++) {
            for (index=0; index < id.length - 1 - counter; index++) {
                if (id[index] > id[index+1]) {
                    temp = id[index];
                    id[index]=id[index+1];
                    id[index+1]=temp;
                }
            }
        }
    }

    public static void printValues (int[]array) {
        
        System.out.println ("\nSorted Id Number: ");
        for(int i = 1; i < array.length; i++){
            System.out.print ("\n" + array[i]);            
        }
            
    }

    public static void printName (String[]array,int a) {
        
        for (int i = 0; i <= a - 1; i++) 
        {
            System.out.print(array[i] + ", ");
        }
            
    }

   public static void nameSort(String[] name,int a)
    {
        String temp;
        for (int i = 0; i < a; i++) 
        {
            for (int j = i + 1; j < a; j++) { 
                if  (name[i].compareTo(name[j])>0) 
                {
                    temp = name[i];
                    name[i] = name[j];
                    name[j] = temp;
                }
            }
        }
    }






}

The sorting works on the id but i have problems in my names it wont push through the bubble sort and say its null, im just starting learning this language and it would be a great help. Ive been working on this since last night and ive tried transffering it under the if else(choice == 2) still it says null.

Choose a number of students: 2 Enter ID number 1: 123 Enter Student name 1: Mark JAw Enter Student course 1: JSJS ---------------------------------------- Enter ID number 2: 221 Enter Student name 2: Ak akw Enter Student course 2: jdj ----------------------------------------

1. Sort by ID
2. Sort by Name
3. Search by ID
4. Search by Name
5. Search by Course
6. Display Records In table Form.
Your Choice: 1

Sorted Id Number:

123
221
1. Sort by ID
2. Sort by Name
3. Search by ID
4. Search by Name
5. Search by Course
6. Display Records In table Form.
Your Choice: 2
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.compareTo(String)" because "name[i]" is null
        at Search.nameSort(Search.java:95)
        at Search.main(Search.java:41)
PS C:\Users\Bingus\Documents\Projects> 

Answer 1

String name[] = new String[n+1];

Suppose for example that you read in a value of 3 for n. This code means that you will allocate 3+1 = 4 elements in name (and likewise in the other arrays). You do not want to allocate 4 elements. You will read 3 names, so you want to allocate 3 elements.

The valid indices for your array will be 0, 1, 2 and 3. You do not want 3 to be a valid index for your array. You want only 0, 1 and 2 to be valid indices, which if you count, you will notice makes 3 different indices.

for(int i=1;i <= n; i++)

In this loop, you will use values of 1, 2 and 3 for i, and thus assign to indices 1, 2 and 3 of name (as well as the other arrays). You do not want to do this. The result is that name[0] will remain null. You want to start the loop at 0 so that you use every element of the arrays. You want to use i < n as your loop condition, because once you correctly only have n elements in the array, n is no longer a valid index.

    for (int i = 0; i < a; i++) 
    {
        for (int j = i + 1; j < a; j++) { 
            if  (name[i].compareTo(name[j])>0) 

When the exception happens, it happens because you correctly start scanning the array from the beginning, but have incorrectly filled (and sized) the array. You pull a null value out of the array (that shouldn't be there) and try to .compareTo another value (which doesn't work, because you can't call a method on a null). A similar problem would occur (if you got that far) in your other sorting method.

(Unless it is for an assignment, you should not implement sorting yourself. You should use java.util.Arrays.sort.)