Why do Calendar.after(...) and before(...) take an Object?

[Jesper de Jong]

Class java.util.Calendar has two methods that you can use to see if a calendar is currently set to a date and time after or before that of another Calendar:

public boolean after(Object when)
public boolean before(Object when)

For some strange reason, these methods take an Object as a parameter. According to the Javadoc, that object must be a Calendar. So why is the parameter type Object instead of Calendar?

Is this just a major crazy design bug in the API or is there a reason for this?

[Paul Sturrock]

Java and anything to do with dates just seems to be badly done. I can't think of a good reason. But then maybe I'm just bitter after spending an afternoon fixing jdk1.5 introduced ClassCastExceptions in TimeStamp's compareTo method.

[John Kelty]

I suppose it could be so that developers could write subclasses of Calendar that work with various data types, maybe Date, or maybe some proprietary class, without breaking the Calendar.before(Object when) method signature.

That doesn�t make it right, but that�s my guess as to why they did it that way.

[Chris Rudd]

I would tend to agree with John here - especially since the methods were previously abstract.

[Jesper de Jong]

Ok, but it doesn't really make sense, or at least, it's not type safe.

Suppose I'd write my own Calendar class, with an after(...) method that would be able to accept a Calendar as well as a Date. The most straightforward and type safe way would be to simply overload the after(...) method:

public class Calendar {
    // Suppose the Calendar API looked like this
    public boolean after(Calendar c) {
        // ...
    }
}
 
public class MySpecialCalendar extends Calendar {
    // Version that takes a Calendar
    public boolean after(Calendar c) {
        // ...
    }
 
    // Version that takes a Date
    public boolean after(Date d) {
        // ...
    }
}

If I want to do this with the current Calendar API, I'm forced to write my after(...) method in a non-OO and non-type safe way:

public boolean after(Object o) {
    if (o instanceof Calendar) {
        // ... do comparison
    }
    else if (o instanceof Date) {
        // ... do comparison
    }
    else {
        throw new IllegalArgumentException("Can't compare with object of type: " + o.getClass().getName());
    }
}

Ugly!! Bad!!