Java 15 Solution using StringBuilder class:

import java.io.*; import java.util.*;

public class Solution {

public static void main(String[] args) {
    StringBuilder builder = new StringBuilder();

    Scanner scan = new Scanner(System.in);

    int i = 1;
    while(scan.hasNext()) {
        builder
        .append(i)
        .append(" ")
        .append(scan.nextLine())
        .append("\n");
        i++;
    }

    scan.close();

    System.out.print(builder.toString());
}

}