How to login as root in your shell using bash script ?

Whenever I need to login as root in bash I use su command.
What if I need to login as root using a bash script?? After the command bash prompts for password.

I tried to write a bash script for the same but could not. I did some googling again got no solution. Then I remembered about a shell utility Expect. I installed it and wrote some simple scripts.

spawn su
expect "Password: "
send "rootpassword\r"
expect "#"
interact

With this script we are left logged into the root and we can run any bash commands there after.

I wrote the next one by mixing bash and expect.

#!/bin/bash
/usr/bin/expect << EOD
set timeout 120
spawn su
expect "Password: "
send "rootpassword\r"
expect "#"
EOD
echo "\r"
ls

But with this we are logged out (we can't interact in the shell as root) after completion of the script.

How to get top n and bottom n lines from a text file in a single pass ?

Some days ago, I came across a problem. The problem was to get the row-count , top n and bottom n number of lines from a text file in a single pass.
I thought for a while and remembered about a data-structure Queue that I had learned about in Data Structures and Algorithm course. I then wrote a simple Java code to get implementing Queue.
Below is the code I wrote, please read and give me feedbacks.

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.LinkedList;
import java.util.Queue;

public class Test {
 private static Queue top=new LinkedList();
 private static Queue bottom=new LinkedList();
 private static int count=0;
 
 public static void main(String[] args) throws IOException{
  func(3);
  }
 
 //function to get count, top n, bottom n lines
 private static void func(int n) throws IOException{
  FileInputStream fstream = new FileInputStream("abc.txt");
  BufferedReader br = new BufferedReader(new InputStreamReader(fstream));

  String strLine;

  //Read File Line By Line
  while ((strLine = br.readLine()) != null){
    count++;
    if(count<=n){
     top.add(strLine);    //initialize both top and bottom as top n 
     bottom.add(strLine);
    }else{
     bottom.remove();
     bottom.add(strLine);
        }
  }
  System.out.println(count);
  System.out.println(top.toString());
  System.out.println(bottom.toString());
  br.close();    
 }
}