Why this class need to be abstract in scala? - scala

Question

In java and c#,I can write this:
class Tree {
Tree left;
Tree right;
}
but in scala:
class Tree{
val left:Tree
val right:Tree
}
I need to add abstract for class,or write:
val left:Tree=new Tree()
I can write this:
trait Tree{
val left:Tree
val right:Tree
}
but why if I use class,I "have to"and abstract?I don't think it's a good design
Thanks!

Answer 1

The reason you can write
class Tree {
Tree left;
Tree right;
}
in Java and C# is because the fields are initialized to null by default (or to 0, etc. depending on the type). Scala's designers decided this is a bad idea and you need to initialize them. So the approximate Scala equivalent is
class Tree {
// note, not val
var left: Tree = null
var right: Tree = null
}
which is legal (but probably not something you actually want to use).

Answer 2

but why if I use class,I "have to"and abstract?
You have to mark your class with the abstract keyword because your class is abstract. It cannot possibly be instantiated.
I don't think it's a good design
Good design is subjective. The designers of Scala thought that being explicit in this case, rather than making the class automatically abstract was good design. I would guess that the majority of the Scala community agrees with them.
You disagree, and that is perfectly okay.

Answer 3

It happened because scala sometimes considers val and def as method like declarations. Let me elaborate, please, for instance, we have the next Scala example:
class Example {
val a: String = "example"
}
val example = new Example()
println(example.a)
You may see that a declared as a field. Unlike Java, we can change this declaration easily to def and everything remains compiling:
class Example {
def a: String = "example"
}
val example = new Example()
println(example.a)
In the case of Java (not sure about C# I've never worked with it), if you would like to access the field over getter method, you will need to change all invocation places from field to method access.
Now, you can consider val as let's say sort of eager cached def version - if so then declaring val without actually value assignment implicitly considered by compiler as declaring method without implementation and that's why compiler says Tree is an abstract class - because left and right has no values, hence they are abstract. In order to make it non-abstract, you need to assign a value to fields or use var if you would like to proceed with mutable structure, e.g.:
class Example {
val a: String = "example"
}
val example = new Example()
println(example.a)
Scatie: https://scastie.scala-lang.org/bQIcVNk9SN6qJhbL32SMUQ