list search and replace huge record

I have a situation as below. If I use usual for loop, it is very time consuming. I have a list of UserRating. Structure of UserRating is as below.

public class UserRating {
    private String userId;
    private String productId;
    private int rating;

      // getters and setters    
}

Example of data for UserRating (userId, productId, rating in the order)

U1, P1, 1
U1, P2, 3
U2, P2, 5
U7, P1, 2
U7, P5, 4

I want to replace the alpha-numeric userId and productId with integer starting from 1. I want to achieve the below output for above data. The rating should be kept same as it is.

1, 1, 1
1, 2, 3
2, 2, 5
3, 1, 2
3, 3, 4

The most important point: I am processing about 5.5 million records. I have already these records in csv file. I read it using java 8 stream API and keep it as `list. Main code where search and replace happens: userTemp and userTemp2 have same elements. Actually in this code, I am just trying to do the replacement for a single column in csv (e.g userId).

for (String original : userTemp) {
    cnt1++;
    if (cnt1 % 10000 == 0) {
        System.out.println(cnt1 + " processed");
    }
    if (userTemp2.contains(original)) {
        countI++;
        Collections.replaceAll(userTemp2, original, String.valueOf(countI));
    }
 }