Java: Why won't this function work without a kinda crappy workaround?

Question

This is a simple function to reverse a string.

If I run it as is it will go like this,
Input: "abc"
Output: "nullcba"

private static String reverseString(String input){
String result;
for(int i=input.length()-1; i>=0; i--){
result = (result + input.charAt(i));
}
return result;
}

I have to do something like this to get a clean output? What am I missing?

private static String reverseString(String input){
String result = "";
for(int i=input.length()-1; i>=0; i--){

if (result==null){
result=(""+input.charAt(i));
}else {
result = (result + input.charAt(i));
}
}
return result;
}

Answer 1

Because you did not initialize String <result> in the first implementation; the String <result> is initialized by null. In second implementation you initialized <result> by "" (empty string).