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In my recent posts about using or omitting a "new" keyword in Scala ( "new" keyword in Scala) I was told that the omission comes from the fact that certain classes have companion objects defined with apply method on them. My question is: are we able to tell or is there any general rule to distinguish which classes/objects have a companion object and apply method?

Thanks in advance and sorry of it's a stupid question, but coming from a Java background it is a bit confusing.

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Bober02
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    You could look in the scaladocs. Or if you use an IDE the code completion should show it. – drexin Mar 20 '12 at 20:54
  • I am using eclipse, how should it tell me whether I am using a companion or not? – Bober02 Mar 21 '12 at 13:49
  • Your question wasn't wether or not you should use a companion, but how to distinguish which classes have a companion with apply defined and the IDE shows you either a C for class, or an O for object in the code completion listing. – drexin Mar 21 '12 at 14:52

2 Answers2

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In the Scala API documentation, you see a little icon enter image description here in the list on the left side. If you click on that, you go to the documentation of the companion object of the class or trait that's mentioned.

Most of the collection classes and many other classes in the standard library have companion objects with apply methods.

For case classes, the Scala compiler automatically creates a companion object with an apply method (as well as other methods).

Jesper
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In the scala API documentation, you can access the companion object by clicking on the icon Object icon as explained by Jesper, but you can also switch between a class and its companion object by clicking on the class/object symbol on the top of the main panel:

enter image description here

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Nicolas
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