java.util.Date clone or copy to not expose internal reference

java clone
Ralph picture Ralph · Aug 16, 2011 · Viewed 42.3k times · Source

It is best practice not to expose the internal references of an Object (Entity). So if an Object has a field of type java.util.Date then for example the getter for this field should return not the original date but an copy of it.

But for an java.util.Date there are two common ways to create that copy:

  • clone: (Date) originalDate.clone()
  • copy via constructor new Date(originalDate.getTime())

My question is, which way is better, and why?

Answer

Jon Skeet picture Jon Skeet · Aug 16, 2011

If it's definitely just a Date, it won't make any difference either way.

If the actual object might be a subclass of Date (such as java.sql.Date) then I'd hope that clone() would preserve the extra information (including which class it is) whereas calling the constructor wouldn't.

As an aside, if you used Joda Time you wouldn't have this problem, as there are plenty of immutable types to use. It's also a much better API :)

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