java - Not sure why I'm getting a NullPointerException error -

so i'm running program various things string array. 1 of them inserting string inside array , sorting it. i'm able use sort method, when attempt insert string , sort nullpointerexception. 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 inserting?");                     first.insertioninorder(insert);                     first.display();                 }//not working                 break;                  }//switch menu         }//main     }//list_driver 

uwb basic util driver. doesn't have problems. listwb file itself:

    public class listwb     {     public void insertionsort()         {             for(int = 1; < 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++;             (int x = size -1; x> index; x--)                 list[x] = list[x-1];             list[index] = str;         }         else              system.out.println("capacity reached");     }//insertioninorder }//listwb 

how fix this?

you have array of 5 strings, 3 of them initialized. rest points null (because 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 initializes array itself, not containing objects.

but insert iterates on 5 elements. , temp null, when 3. statement temp.compareto throws nullpointerexception.

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

solution: check temp null in while loop. or not use string array @ auto-resizable data structure list java.util.arraylist.