Variable inside if used outside [duplicate]

This question already has an answer here:

• Using a variable defined in an If statement? [duplicate] 6 answers

What i'm trying to to is to get the first char of string a, and last char of string b. Empty a should return "@+lastb" and empty b should return "firsta+@".

examples: a = "hello", b = "hi" should return "hi"; a = "" and b = "hi" returns "@i";

public String lastChars(String a, String b) {
 if(a.length() > 0) {
   String firstA = a.substring(0,1);
 }
 else {
   String firstA = "@";
 }
 if(b.length() > 0) {
   String lastB = b.substring(b.length()-1);
 }
 else {
   String lastB = "@";
 }
 return firstA + lastB;
}

the error message i get is that the variables cannot be resolved, which im guessing means that they never were made?