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public class BookWrapper {
  private final Book book;
  private final Object lock = new Object();

  BookWrapper(Book book) {
    this.book = book;
  }

  public void issue(int days) {
    synchronized(lock) {
      book.issue();
    }
  }

  public Calendar getDueDate() {
    synchronized(lock) {
      return book.getDueDate();
    }
  }

  public void renew() {
    synchronized(lock) {
      if (book.getDueDate().after( Calendar.getInstance())) {
        throw new IllegalStateException("Book overdue");
      } else {
        book.issue(14);
      }
    }
  }
}