Write a program in Java to accept a string. Arrange all the letters of the string in an alphabetical order. Now, insert the missing letters in the sorted string to complete all the letters between first and last characters of the string.

Sample Input:
computer

Alphabetical order:
cemoprtu

Sample Output:
cdefghijklmnopqrstu

import java.util.Scanner;

public class KboatStringSort
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.println("Enter a string:");
        String str = in.nextLine();
        str = str.toLowerCase();
        int len = str.length();

        String sortedStr = ""; //Empty String
        for (char ch = 'a'; ch <= 'z'; ch++) {
            for (int i = 0; i < len; i++) {
                char strCh = str.charAt(i);
                if (ch == strCh) {
                    sortedStr += strCh;
                }
            }
        }
        
        System.out.println("Alphabetical order:");
        System.out.println(sortedStr);
        
        String filledStr = ""; //Empty String
        for (int i = 0; i < len - 1; i++) {
            char strCh = sortedStr.charAt(i);
            filledStr += strCh;
            for (int j = strCh + 1; j < sortedStr.charAt(i+1); j++) {
                filledStr += (char)j;
            }
        }
        filledStr += sortedStr.charAt(len - 1);
        
        System.out.println("Filled String:");
        System.out.println(filledStr);
    }
}