Refactor listBooks method

Extract instance variable from local variable in listBooks method
and eliminate duplicated code by adding initializeBooks.

src/com/twu/biblioteca/Biblioteca.java

@@ -11,21 +11,24 @@ public class Biblioteca {
     List<Book> books = new ArrayList<Book>();
 
     public Biblioteca() {
+        initialiazeBookList();
     }
 
     public Biblioteca(String welcomeMessage) {
-
         this.welcomeMessage = welcomeMessage;
+        initialiazeBookList();
     }
 
     public String welcomeUser() {
         return welcomeMessage;
     }
 
     public List<Book> listBooks() {
-        List<Book> bookList = new ArrayList<Book>();
+        return books;
+    }
+
+    private void initialiazeBookList() {
         Book book = new Book("Harry Potter");
-        bookList.add(book);
-        return bookList;
+        books.add(book);
     }
 }

src/com/twu/biblioteca/BibliotecaApp.java

@@ -5,5 +5,9 @@ public class BibliotecaApp {
     public static void main(String[] args) {
         Biblioteca biblioteca = new Biblioteca();
         System.out.println(biblioteca.welcomeUser());
+
+        for(Book book : biblioteca.listBooks()) {
+            System.out.println(book.name + "\n");
+        }
     }
 }