“new” keyword in Scala

Question

I have a very simple question - when should we apply the new keyword when creating objects in Scala? Is it when we try to instantiate Java objects only?

Answer 1

Is it when we try to instantiate java objects only?

Not at all. There is two general cases when you ommit new in scala. With singleton objects (that are oftenly used to store static functions and as a kind of factory similar to what you may seen in java):

scala> object LonelyGuy { def mood = "sad" }
defined module LonelyGuy

scala> LonelyGuy
res0: LonelyGuy.type = LonelyGuy$@3449a8

scala> LonelyGuy.mood
res4: java.lang.String = sad

With a case classes (actually, underneath there are class + object = companion pattern, e.g. having class and object with the same name):

scala> case class Foo(bar: String) 
defined class Foo


scala> Foo("baz")
res2: Foo = Foo(baz)

So when you work with a simple classes, rules are the same as with Java.

scala> class Foo(val bar: String) 
defined class Foo

scala> new Foo("baz")
res0: Foo = Foo@2ad6a0

// will be a error 
scala> Foo("baz")
:8: error: not found: value Foo
       Foo("baz")

Bonus, there is a anonymous classes in scala, which can be constructed like this:

scala> new { val bar = "baz" }
res2: java.lang.Object{val bar: java.lang.String} = $anon$1@10ee5b8

scala> res2.bar
res3: java.lang.String = baz