Why don't Java for loops work the same way?

public static int[] plusOne(int[] digits) {
   long n = 0;
   for(int i = 0; i < digits.length; i++) {
     n = n*10+digits[i];
   }
   System.out.println(n);
   n = 0; 
   for(int i : digits) {
     n = n*10+digits[i];
   }
   System.out.println(n);
   return digits;
}

The above code for me prints entirely different numbers for n depending on which loop I use. The first one prints out "9876543210" and the second one prints out "123456789". My input array is "{9,8,7,6,5,4,3,2,1,0}." I am changing the int[] array to a single value and expected output is the result of the classic for loop "9876543210". My understanding is that these should work essentially the same way.

This line will count indexes for you:

for(int i = 0; i < digits.length; i++)

and you have to get to the real number by looking it up:

int digit = digits[i];
n = n*10+digit;

While that expression will directly give you the numbers you are interested in:

for(int digit : digits) {

so you do not look up but use it directly:

n = n*10+digit;