Why some java API classes are not final?

[Priti Ksharma]

Dear all,

I was just wondering that we hardly extend some classes like ArrayList or classes from Collection API or IO stream classes, I actually never extended these. Why these classes are not made final? Any specific reason?

Thank you.

[Saifuddin Merchant]

Priti Ksharma wrote:Dear all,

I actually never extended these

Thank you.

Maybe there are others who need to extend it besides you?

Direct Known Subclasses:
AttributeList, RoleList, RoleUnresolvedList

[Priti Ksharma]

Sam, can you give any example where we will need to extend these classes?

[Jeff Verdegan]

Priti Ksharma wrote:Sam, can you give any example where we will need to extend these classes?

Why does it matter? All that really matters is the designers considered that there might be some use to somebody to be able to extend them, and there was no harm in doing so, so there was no need to make them final.

And Sam gave you examples of 3 classes in the core API that extend ArrayList. What more do you need?

[Campbell Ritchie]

Welcome to the Ranch

Library classes are usually designed differently from ordinary classes. In ordinary classes, you try to make the public interface as small as possible, only providing something public if you can think of a good reason for it.
In library classes, you tend to provide things public just in case anybody wants them. You might only make a library classes un-extendable if they are to be immutable.