Why is the Java date API (java.util.Date, .Calendar) such a mess?

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Someone put it better than I could ever say it:

  • Class Date represents a specific instant in time, with millisecond
    precision. The design of this class is a very bad joke – a sobering
    example of how even good programmers screw up. Most of the methods in
    Date are now deprecated, replaced by methods in the classes below.

  • Class Calendar is an abstract class for converting between a Date
    object and a set of integer fields such as year, month, day, and hour.

  • Class GregorianCalendar is the only subclass of Calendar in the JDK.
    It does the Date-to-fields conversions for the calendar system in
    common use. Sun licensed this overengineered junk from Taligent – a
    sobering example of how average programmers screw up.

from Java Programmers FAQ, version from 07.X.1998, by Peter van der Linden – this part was removed from later versions though.

As for mutability, a lot of the early JDK classes suffer from it (Point, Rectangle, Dimension, …). Misdirected optimizations, I’ve heard some say.

The idea is that you want to be able to reuse objects (o.getPosition().x += 5) rather than creating copies (o.setPosition(o.getPosition().add(5, 0))) as you have to do with immutables. This may even have been a good idea with the early VMs, while it’s most likely isn’t with modern VMs.

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