// CSE 143, Summer 2016
//
// A handy example of keeping track of web pages visited.

import java.util.*;

// An example showing a back button used in a browser. A perfect
// application for a stack, a "last in, first out (lifo)" data
// structure.
public class BackButton {
  public static void main(String[] args) {
    Stack<String> pages = new Stack<String>();
    
    visit("google.com", pages);
    visit("reddit.com", pages);
    
    System.out.println(pages);
    System.out.println();
    
    goBack(pages);
    System.out.println(pages);
    goBack(pages);
  }
  
  // pre : pages should not be null (throws IllegalArgumentException otherwise)
  // post: adds page to pages, and pronounces the page as being visited
  public static void visit(String page, Stack<String> pages) {
    if (pages == null) {
      throw new IllegalArgumentException();
    }
    System.out.println("Visiting " + page + "...");
    pages.push(page);
  }
  
  // pre : pages should not be null (throws IllegalArgumentException otherwise)
  // post: takes a page from the top of pages, and says that the page is being
  //       left. Also prints the current page, if there is one left.
  public static void goBack(Stack<String> pages) {
    if (!pages.isEmpty()) {
      // Someone in class was talking about the one line version.
      // The below is the equivalent one line version.
      // ---
      // System.out.println("Leaving " + pages.pop() + "...");
      String oldPage = pages.pop();
      System.out.println("Leaving " + oldPage + "...");
      if (!pages.isEmpty()) {
        // The below is equivalent.
        // ---
        // String currentPage = pages.pop();
        // System.out.println("Currently on " + currentPage);
        // pages.push(currentPage);
        // ---
        String currentPage = pages.peek();
        System.out.println("Currently on " + currentPage);
      } else {
        System.out.println("No page left!");
      }
    } else {
      System.out.println("No page!");
    }
  }
}