Why does implement abstract method using val and call from superclass in val expression return NullPointerException

Question

I have an abstract class with an unimplemented method numbers that returns a list of numbers, and this method is used in another val property initialization:

Answer 1

Here is what your code attempts to do when it initializes MainFoo:

1. Allocate a block of memory, with enough space for val calcNumbers and val numbers, initially set to 0.
2. Run the initializer of the base class Foo, where it attempts to invoke numbers.map while initializing calcNumbers.
3. Run the initializer of the child class MainFoo, where it initializes numbers to List(1, 2, 3).

Since numbers is not initialized yet when you try to access it in val calcNumbers = ..., you get a NullPointerException.

Possible workarounds:

1. Make numbers in MainFoo a def
2. Make numbers in MainFoo a lazy val
3. Make calcNumbers in Foo a def
4. Make calcNumbers in Foo a lazy val

Every workaround prevents that an eager value initialization invokes numbers.map on a non-initialized value numbers.

The FAQ offers a few other solutions, and it also mentions the (costly) compiler flag -Xcheckinit.

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