谁来检查方法参数合法性

public static void main(String[] args) {
   Book book = null;
   new Main().buy(book);
}


public void buy(Book book) {
   System.out.println(book.getPrice());
}

public static void main(String[] args) {
   Book book = null;
   if (book != null) {
       new Main().buy(book);
   }
}

public void buy(Book book) {
   if (book != null) {
       System.out.println(book.getPrice());
   }
}

/**
* bub a book
* @param book could be null
*/

public void buy(Book book) {
   if (book != null) {
       System.out.println(book.getPrice());
   }
}