Encrypt / Decrypt homework help

Hello
I got a hw assignment in which I have to ask the user weather they want to encrypt or decrypt. then it asks them a value of N
to do the encrypt I take the ASCII value and add N to it. Finaly I ask them a sentence.

all has gone well except one issue. In this class, code follows, I have the methods to en/decrypt the text. when I run I get a out of memory error on the "sbb.append(aChar);" line

Thanks for any and all help!


import java.text.*;
import java.lang.*;
import java.io.*;
import javax.swing.*;

public class Calculator {

private String choice;
private int N;
String results;
char aChar;
String test;

public void setN(int n){
N = n;
}

public void encrypt(String e){

StringBuffer sbb = new StringBuffer("");

choice = e;
StringBuffer sb = new StringBuffer(choice);
int length = choice.length();

int i = 0;
while( i < length){
aChar = choice.charAt(i);
aChar += N;
sbb.append(aChar);
}
test = sbb.toString();
}

public void decrypt(String d){

StringBuffer sbb = new StringBuffer("");

choice = d;
StringBuffer sb = new StringBuffer(choice);
int length = choice.length();

int i = 0;
while( i < length){
aChar = choice.charAt(i);
aChar -= N;
sbb.append(aChar);
}
test = sbb.toString();

}


public String sendDisplay(){
results = test;
return results;
}

}

You might want to consider doing something with "i", i++ would probably work

Good luck.

once again I turn something that is way to easy i++ and forget it

thanks