Java Program: Web Browser History

Objective

 Using LinkedList or Stack class 

Background

In a modern web browser, web pages visited are stored in a browsing history and the user can navigate to any page in the browsing history by using the “forward” and “back” buttons on the web browser. In this task, you are to implement a simple simulation of these operations. Note: You are required to make use of the LinkedList ADT or any of its variants, or the Java API Stack, in solving this problem. 

Task statement

The behaviours of the “forward” and “back” buttons in a web browser are described as follows. A user can browse a page by entering a web address, known as the URL (Universal Resource Locator). The URL entered is stored in a browsing history list. 

The user may also click the “forward” or “back” buttons to navigate to a different page in the browsing history. At any time, when the “back” button is clicked, the browser is redirected to the previous page that the user visited. The “back” button can be clicked many times to redirect the browser to a page further back in history. When the browsing history is empty or the current page is the earliest page in the browsing history, clicking the “back” button will do nothing. 

The “forward” button exhibits the opposite behaviour to the “back” button. When clicked, the browser redirects to the next URL in the browsing history. Clicking the “forward” button will do nothing if the browsing history is empty or the current page is the latest page in the browsing history. 

Note that each time the user navigates to a new page by entering a URL, the URLs in the browsing history that come after the current URL are cleared. An example is given below. 

Browsing history (ordered from earliest to latest): 
http://www.google.com 
http://www.gmail.com <- current page user is browsing 
http://www.comp.nus.edu.sg 
http://www.yahoo.com   

If the user then enters a new URL – www.nus.edu.sg, the browsing history is changed as follows: 
http://www.google.com 
http://www.gmail.com 
http://www.nus.edu.sg <- current page 

Input

The input consists of some lines, where each line consists of exactly one of the following 3 types of instructions.

• If the line consists of the word “BACKWARD”, it indicates the “back” button has been clicked.
• If the line consists of the word “FORWARD”, it indicates that the “forward” button has been clicked. 
• Otherwise, the line consists of a URL that the user has entered. 

Note that the number of lines is not indicated in the input. An example input is given below. 

http://www.google.com 
http://www.hotmail.com 
BACKWARD 
FORWARD 
http://www.comp.nus.edu.sg 
https://ivle.nus.edu.sg/ 
BACKWARD 
BACKWARD 
http://uva.onlinejudge.org/ 
BACKWARD

Output

The final browsing history at the end of the simulation ordered by earliest page to latest page, as well as the current page that is being browsed. The output for the example input above is given below: 

Browsing History: 
http://www.google.com 
http://www.hotmail.com 
http://uva.onlinejudge.org/ 
Current Page: 
http://www.hotmail.com  

If the browsing history is empty, simply print: Browsing history is empty. 

Solution

import java.util.*;

/* WebBrowser Simulator */
public class WebBrowser {
 Stack <String> backward = new Stack <String>();  // Backward pages
 Stack <String> forward = new Stack <String>();   // Forward pages

 public WebBrowser(Scanner sc) {
  while (sc.hasNext()) {
   String input = sc.nextLine();
   
   switch(input) {
    case "BACKWARD":
    if (!backward.empty())
     forward.push(backward.pop());  // Remove from backward, add to forward
    break;

    case "FORWARD":
    if (!forward.empty())
     backward.push(forward.pop());  // Remove from forward, add to backward
    break;

    default:
    backward.push(input);      // New page opened
    while (!forward.empty())   // Clear forward stack
     forward.pop();
    break;
   }
   if (backward.empty() && !forward.empty()) // Add forward pages to history
    backward.push(forward.pop());
  }

  if (backward.empty() && forward.empty())
   System.out.println("Browsing history is empty.");

  else {

   // If there is only 1 page, current page in forward stack else in backward
   String current = (backward.empty()) ? forward.peek() : backward.peek();

   System.out.println("Browsing History:");

   while (!forward.empty())       // In order to print from least to most recent
    backward.push(forward.pop());

   while (!backward.empty())
    forward.push(backward.pop());

   while (!forward.empty())
    System.out.println(forward.pop());
  
   System.out.println("Current Page:");
   System.out.println(current);
  }
 }

 public static void main(String[] args) {         
  Scanner sc = new Scanner(System.in);     
  WebBrowser bc = new WebBrowser(sc);

 }

}