how to force method to be implemented in concrete subclass from trait

Question

I have a method in my trait

def controller: AnyRef

but my concrete class was not implementing that method and it was still compiling. The compiler doesn't let me add abstract to that method either. How can I create a method in a trait that forces it's implementer to implement it?

thanks, Dean

Answer 1

The compiler enforces that concrete classes implement all the abstracts methods they inherit from superclasses and traits.

If your class was compiling it meant it wasn't concrete, i.e. it was a trait or an abstract class, and you can't force neither to implement the abstract method.

Of course, as soon as you try to obtain a concrete instance the compiler will raise an error as the method is not implemented.

Practical example in the REPL

scala> trait A { def controller: AnyRef }
defined trait A

scala> trait B extends A
defined trait B

scala> abstract class C extends A
defined class C

scala> class D extends A
<console>:8: error: class D needs to be abstract, since method controller in trait A of type => AnyRef is not defined
       class D extends A

scala> new B { }
<console>:10: error: object creation impossible, since method controller in trait A of type => AnyRef is not defined
              new B { }
                  ^

scala> new C
<console>:10: error: class C is abstract; cannot be instantiated
              new C