android - how to replace String array value with another value in java -

string[] arr_infraid;    list<string> list_infraid = new arraylist<string>();  arr_infraid = list_infraid.toarray(new string[list_infraid.size()]);  

this string array , values in string array getting 1,2,3,4...10 respectively want replace values a,b,c,.....j respectively , store in string array 1->a,2->b,3->c........10->j. please tell me how replace getting null pointer exception when trying got loop , try replace it.

public class main { static string [] string1 = {         "1","2",         "3","4",         "5","6",         "7","8",         "9","10"};  static arraylist<string> = new arraylist<string>(); public static void main(string[] args) {      for(int =0;i<string1.length;i++){         getarray(string1[i]);     }     system.out.println(as.tostring()); }  public static void  getarray(string string){     switch (string) {     case "2":         as.add("a");         break;     case "3":         as.add("b");         break;     case "4":         as.add("c");         break;     case "5":         as.add("d");         break;     case "6":         as.add("f");         break;     case "7":         as.add("g");         break;     case "8":         as.add("h");         break;     case "9":         as.add("i");         break;     case "10":         as.add("j");         break;      default:         break;        }     } }