Unexpected output using ,while loop and hasNext() in java

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

public class Solution {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int i=1;
        while(scan.hasNext()){
            System.out.println(i + " " + scan.nextLine());
            i++;
        } 
    }
}

OUTPUT:

a
1 a
b
2 b
c
3 c

e
4 
5 e

PROBLEM : when i have entered , a the output is what it is expected , for b and c is also the same.
but i press enter instead of d the expected out put is :"4 " but it doesnt output 4 , instead no output is given and when i input e after press enter both 4"space" and 5 e are printed. Can someone please point what i am missing to see here?

This happens because you are using the nextLine method. When you press <kbd>Enter</kbd> without entering a character, hasNext() waits until the next character. When you enter e and then press <kbd>Enter</kbd>, hasNext() returns. But you have now entered two lines of text: one empty, and one with "e". The call to nextLine returns the empty line, and on the next run of the loop it returns the line with "e".

To stop this from happening, don't use a mixture of hasNext and nextLine, use either hasNext/next or hasNextLine/nextLine. The difference is that the first counts "words" separated by whitespace, the second counts lines.

    while(scan.hasNextLine()){
        System.out.println(i + " " + scan.nextLine());
        i++;
    } 

nextLine() of scanner past the current line and return the input that was skipped, it's why it counts every time you enter down without inputs, if you want to avoid the skip, you can try scan.next()