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());
}

}

import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

public class Solution {

public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);

    for(int i=1; sc.hasNext(); i++){
        System.out.println(i+" "+sc.nextLine());
    }
    sc.close();
}

}

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import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

public class Solution {

public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);
    int i = 1;
    while(sc.hasNext()) {
        System.out.println(i + " "+ sc.nextLine());
        i++;
    }
    sc.close();
}

}

    Scanner x= new Scanner(System.in);
    int i = 1;
    while(x.hasNext()){
        System.out.printf("%d %s%n", i++, x.nextLine());
    }
            x.close();
            don't forget close scanner