我应该使用哪种方法？

Question

I'm an intermediate java coder, writing a librarian Java app, and for some reason I just can't figure out whether I should use dueDate.after(today) OR dueDate.before(today) method as to deciding if the book is overdue. I've gotten quite some contradictory values by typing both the methods. Hence, I'm also assuming there is some other bug in my code as well, so it would be nice if you can conform which is the correct method so that I can move on the fixing the other bug.

Answer 1

You need dueDate.before(today) : the due date is before today; the due date is in the past, so the book is past due.

Maybe it's easier if you swap the objects around? You'd get today.after(dueDate) and if you read that out loud, it suddenly becomes quite clear: "if today is after the due date, then ..."

Answer 2

Remember that the before and after methods perform a < (or > ) comparison, rather than a <= (or >= comparison). That is what is meant by the word "strictly" in the API documentation.

Also, Java Date objects are really an instant in time, rather than what people usually think of as a "date". That is, it won't just compare the day, but the time of day.

If you want to compare only the day, without checking the time during that day, create all of your due dates to be at a specific time, like midnight. For example, suppose that a book is due October 26. The due date could be midnight, October 27.

boolean overdue = !now.before(dueDate);

The somewhat awkward negation accounts for the case when it is now exactly 12:00 AM Oct. 27.

Answer 3

dueDate.before(today) would translate to the actual dueDate of the book occurring before today, meaning that it is actually overdue. This is probably what you want (assuming you are checking for true)

the other side of things

dueDate.after(today) would translate to the actual dueDate of the book occurring after today, meaning that it is not yet over due.

Answer 4

The only difference between !dueDate.after(today) and dueDate.before(today) is the result when both dates are exactly the same - presumably a book is not overdue when returned on the due date, so dueDate.before(today) should be correct.

As for your unspecified other problems: are you aware that java.util.Date represents a millisecond-precision point in time, not a calendar date? That means that in order to use its comparison methods for calendar dates, you have to make very sure you set the time components to zero when creating your Date instances. Another cause of problems could be time zone differences.

Answer 5

It depends on what you want to achieve. Is dueDate the date you want to set when lending a book? The due date is derived from the lending date of the book, so I would try an approach like book.isDue(today) assuming that the book object contains the lending and due dates as attributes.

With questions like this it helps if you make it explicit for yourself which objects are involved and what their relations are before you design the actions between the objects.