Does Scala have object initialization like C#?

Question

Object initialization in C#:

var foo = new Foo() { bar = 5 };

which is equivalent of

var foo = new Foo();
foo.bar = 5;

Does Scala have object initialization like C#?

Answer 1

scala> class Foo { var bar: Int = _ }
defined class Foo

scala> var foo = new Foo() { bar = 5 }
foo: Foo = $anon$1@1ed00d1

scala> var realFoo = new Foo()
realFoo: Foo = Foo@1bedb0

You can see that the syntax works (in this case), but also that foo does not have type Foo. It's actually a new subtype of Foo which subclasses it with whatever is between the braces. In this case just the constructor with an additional assignment.

In the general case, the exact C# syntax will not work:

scala> class Foo { var bar: Int = _; var baz: String = _ }
defined class Foo

scala> var foo: Foo = new Foo { bar = 5, baz = "bam" }
<console>:1: error: ';' expected but ',' found.
       var foo: Foo = new Foo { bar = 5, baz = "bam" }

Instead you'll need to type:

scala> var foo: Foo = new Foo { bar = 5; baz = "bam" }
foo: Foo = $anon$1@1be20c

Foo constructor will run first, then the anonymous type constructor will run next.

I think this is what Kim and Alexey comments refer to.

I don't use C#, but it seems in C# this bit of syntax can only be used to initialize fields, while in Scala it was decided to use this syntax to extend the class like in Java.

Answer 2

Scala does not have object initialization like C#, but Scala does not need it most of the time. There are factory methods for collections, which can easily substitute object initialization of C#'s collections. There are case classes with copy methods, which help with the initialization of the rest of the objects. There are anonymous classes, which help extending a class and initializing an instance of a class with all it's members at the same time.