Why is using f.length() in a for loop so inefficient in java?

Question

I was writing a program that uses some file processing using the file class and a Buffered Input and Output Streams and found that when I used the following code

for(long j = 0; j < f2.length(); j++) {
    out.write(in.read());
}

was highly inefficient, taking about 15251 milliseconds. However if I saved f2.length() to a variable first

long y = f2.length();
for(long j = 0; j < y; j++) {
    out.write(in.read());
}

it only takes 74 milliseconds. Why is the second block so much more efficient?

Answer 1

Is it possible that saving f2.length() in a variable y will only compute length() once, and in a for loop it will compute for every iteration, multiplying f2.length() computations n times for n number of iterations.

I am curious too. :)