Enforce Library singleton

src/main/java/edu/uastw/Library.java

@@ -9,7 +9,7 @@ public class Library implements Iterable<LibraryItem> {
     private final List<LibraryItem> libraryItems;
     private int booksCapacity = 3;
 
-    Library() {
+    private Library() {
         this.libraryItems = new ArrayList<>();
     }
 

src/main/java/edu/uastw/LibraryDecorator.java

@@ -1,6 +1,6 @@
 package edu.uastw;
 
-public abstract class LibraryDecorator extends Library {
+public abstract class LibraryDecorator {
     protected Library library;
 
     public LibraryDecorator(Library library) {