Here is my suggestion on how you can improve your code:
char[] nkey = new char[16];
for (int i =0 ;i<16;++i) {
nkey[i] = 65;
}
while (true) {
//... do the stuff you do in the inner of the cycle
int index = 15;
nkey[index]++;
while (index >= 0 && nkey[index] >= 122) {
nkey[index] = 65;
index--;
if (index < 0) {
break;
}
nkey[index]++;
}
}
You can imagine what I do as representing what you iterate upon as a number in base 122-65
and adding one to it.