Processing 1.2.1
I have not used nf() before but this seems like it should work:

1. String frame1;
2. int mill1 = 2;
3. if(mill1 < 10) {
4.   frame1 = nf(mill1, 2); 
   
5. }
   
6. println(frame1);     

However, it returns "The local variable frame1 may not have initialized"

What is strange is that this works:

1. String frame1;
2. int mill1 = 2;
3. if(mill1 < 10) {
4.   frame1 = nf(mill1, 2); 
   
5. } else {
6.   frame1 = Integer.toString(mill1);
7. }

It returns, as expected: 02

Am I misunderstanding nf() or is this a bug?