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Scala abstract method is null in a superclass when a subclass implements it with val?

I found an error in my scala code that puzzled me. The following is a simplified version of the problem.

In the constructor of an abstract class, I want to check out a few statements about abstract methods. Thus, when a subclass object is created, these statements are checked to ensure that everything is implemented as it should.

It is incorrect if a subclass implements an abstract method using "val":

Scala code:

abstract class A {
    def aval : String
    assert(aval != null, "aval == null")
    assert(aval == "B", "aval: "+aval)
}

class B extends A {
    def aval = "B"
}

class C extends A {
    val aval = "B"
}

object VariousScalaTests {
    def main(args : Array[String]) : Unit = {
        val b = new B
        val c = new C
    }
}

Scala Error:

Exception in thread "main" java.lang.AssertionError: assertion failed: aval == null
    at scala.Predef$.assert(Predef.scala:92)
    at A.<init>(VariousScalaTests.scala:4)
    at C.<init>(VariousScalaTests.scala:12)
    at VariousScalaTests$.main(VariousScalaTests.scala:19)
    at VariousScalaTests.main(VariousScalaTests.scala)

Thus, it does not work in the last line of code: "val c = new C". Class B works fine, but class C doesn't work! The only difference is that C implements aval using "val" and B using "def".

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, , scala? , scala?

+5
2

Java- :

public abstract class A {
    public String aval();
}

public class B extends A {
    public String aval() {
        return "B";
    }
}

public class C extends A {
    private String _aval;

    public C() {
        _aval = "B";
    }

    public String aval() {
        return _aval;
    }
}


val c = new C

A C, _aval . , aval() null ( _aval).

val b = new B

.

.

, scala?

. .

+5

Scala , val :

class C extends {
  val aval = "B"
} with A
+6

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