不知道为什么我会收到NullPointerException错误

Question

So I'm running a program that does various things to a String array. One of them is a inserting a string inside the array and sorting it. I'm able to use the sort method, but when I attempt to insert a string and then sort it I get a NullPointerException. This is the code:

    import java.util.Scanner;
    import java.io.*;

    public class List_Driver
    {
        public static void main(String args[])
        {
            Scanner keyboard = new Scanner(System.in);
            int choice = 1;
            int checker = 0;
            String [] words = new String[5];
            words[0] = "telephone";
            words[1] = "shark";
            words[2] = "bob";
            ListWB first = new ListWB(words);
            int menu = uWB.getI("1. Linear Seach\n2. Binary Search\n3. Insertion             in Order\n4. Swap\n5. Change\n6. Add\n7. Delete\n8. Insertion Sort\n9. Quit\n");
            switch(menu)
            {
                //other cases
                case 3:
                {
                    String insert = uWB.getS("What term are you inserting?");
                    first.insertionInOrder(insert);
                    first.display();
                }//not working
                break;

                }//switch menu
        }//main
    }//List_Driver

uWB is a basic util driver. It doesn't have any problems. This is the ListWB file itself:

    public class ListWB
    {
    public void insertionSort()
        {
            for(int i = 1; i < size; i++)
        {
        String temp = list[i];
        int j = i;
        while(j > 0 && temp.compareTo(list[j-1])<0)
        {
            list[j] = list[j-1];
            j = j-1;
        }
        list[j] = temp;
        }
    }
    public void insertionInOrder(String str)
    {
            insertionSort();
        int index = 0;
        if(size + 1 <= list.length)
        {
            while(index < size && str.compareTo(list[index])>0)
                    index++;
            size++;
            for (int x = size -1; x> index; x--)
                list[x] = list[x-1];
            list[index] = str;
        }
        else 
            System.out.println("Capacity Reached");
    }//insertioninorder
}//ListWB

How would I fix this?

Answer 1

You have an array of 5 Strings, but only 3 of them initialized. The rest points to null (because you did not initialize them):

  String [] words = new String[5];
  words[0] = "telephone";
  words[1] = "shark";
  words[2] = "bob";
  words[3] = null;
  words[4] = null;

The first line only initializes the array itself, but not the containing objects.

But the insert iterates over all 5 elements. And temp is null, when i is 3. So the statement temp.compareTo throws an NullPointerException.

 for(int i = 1; i < size; i++)
    {
    String temp = list[i];
    int j = i;
    while(j > 0 && temp.compareTo(list[j-1])<0)

Solution: Also check temp for null in the while loop. Or do not use a string array at all but a auto-resizable data structure list java.util.ArrayList.