title: Removing ArrayList object issue
body: <p>I got an issue with deleting an object from ArrayList when working on the assignment
  If I use the "normal" for loop, it works as following</p>
  
  <pre><code>public void returnBook(String isbn){        
      for (int i = 0; i &lt; booksBorrowed.size(); i++){            
          if (booksBorrowed.get(i).getISBN() == isbn){
              booksBorrowed.get(i).returnBook();
              booksBorrowed.remove(i);                
          }
      }
  }
  </code></pre>
  
  <p>However, when I'm trying to simplify the code with enhanced for-loop, that doesn't work and showing java.util.ConcurrentModificationException error:</p>
  
  <pre><code>public void returnBook(String isbn){        
          for (Book book: booksBorrowed){            
              if (book.getISBN() == isbn){
                  book.returnBook();
                  booksBorrowed.remove(book);                
              }
          }
  }
  </code></pre>
  
  <p>Hope you guys could lighten me up..</p>