Is there something wrong with an abstract value used in trait in scala?

Question

I have

trait Invoker {   val method: Method } 

Intellij IDEA code inspection is warning me that "Abstract value used in trait". Everything compiles fine. Is there something wrong with having an abstract value in a trait? If so, how should I specify that all extenders of the trait must define a method property?

Answer 1

What is meant by this is the following weirdness:

trait A {   val i: String   def j: String }  class C extends A {   println ("val i = " + i)   println ("def j = " + j)    val i = "i"   def j = "j" }  val c = new C // prints // val i = null // def j = j 

So, as you can see i is initialised to it default value (null for AnyRef) before it is finally overridden by the constructor in C. (def declarations are re-referenced immediately.)

To avoid this one would have to put the val initialisations to the beginning of the constructor, if possible.

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Additional weirdness (and how to solve it) in the following case

Consider

trait A {   val i: String   def j: String }  abstract class D extends A {   println ("val i = " + i)   println ("def j = " + j) }  class C extends D {   val i = "i"   def j = "j" } val c = new C // prints // val i = null // def j = null 

Now we seem to be out of luck; it looks as if there is no chance for us to initialise val i and def j before our superclass D tries to print them. In order to solve this problem, we must use Early definitions (§5.1.6 Scala reference):

class C extends {   val i = "i"   def j = "j" } with D  val c = new C // prints // val i = i // def j = j 

And it works!