### Question:
FileShoppingCart implements Serializable { public static FileShoppingCart instance() { return new FileShoppingCart(); } private  FileShoppingCart(); static FileShoppingCart instance(); String serializeShoppingCartContentsToStringOneItemPerLine(); static FileShoppingCart instanceFromSerializedStringRepresentation(final String stringRepresentation); @Override String toString(); boolean hasItems(); void addAnItem(final ShoppingCartEntry shoppingCartEntry); void removeAnItem(final String fileName); List<String> getShoppingCartFileList(); void clearCart();  }### Answer:
@Test public void testInstance() { FileShoppingCart fileShoppingCart = FileShoppingCart.instance(); Assert.assertNotNull("null shopping cart", fileShoppingCart); }