java - NullPointerException initializing variable with string length -

we told implement method detect if string palindrome iteration , recursion. successful in implementing method iteration getting error when try , recursively.

exception in thread "main" java.lang.nullpointerexception @ recursion.recursivepallindrome.ispallindrome(recursivepallindrome.java:14) @ recursion.recursivepallindrome.main(recursivepallindrome.java:44) 

i'm clueless on has gone wrong.

package recursion;  public class recursivepalindrome {  static string word; public recursivepalindrome(string a) {     = word; }  public static boolean ispalindrome() {     int start = 0;     int end = word.length()-1; //line 14         char = word.charat(start);         char z = word.charat(end);             character.tolowercase(a); character.tolowercase(z);      if(start >= end)     {         if(character.isletter(a) && character.isletter(z))         {             if(a == z)             {                 a++;                 z--;             }             else                 return false;         }         else if(!character.isletter(a))             start++;         else if(!character.isletter(z))             end--;         else             return false;     }      return true; }  public static void main(string[] args) {     new recursivepalindrome("testing");     if(ispalindrome())         system.out.println("is!");     else         system.out.println("is not!"); } } 

try:

public recursivepalindrome(string a) {     word = a; } 

this code works, it isn't recursion!

package recursion;  public class recursivepalindrome {      static string word;      public recursivepalindrome(string a) {         word = a;     }      public static boolean ispalindrome() {         int start = 0;         int end = word.length() - 1;         char = word.charat(start);         char z = word.charat(end);         character.tolowercase(a);         character.tolowercase(z);          while (start < end) {             if (character.isletter(a) && character.isletter(z)) {                 if (a == z) {                     start++;                     end--;                     = word.charat(start);                     z = word.charat(end);                 } else {                     return false;                 }             } else if (!character.isletter(a)) {                 start++;             } else if (!character.isletter(z)) {                 end--;             } else {                 return false;             }         }          return true;     }      public static void main(string[] args) {         new recursivepalindrome("testing");         if (ispalindrome())             system.out.println("is!");         else             system.out.println("is not!");     } }