Programming Scala, 3rd Edition

Chapter 12. Instance Initialization and Method Resolution

A type has a directed acyclic graph (DAG) of dependencies with its supertypes. When the fields in a type are spread over this DAG, initialization order can become important to prevent accessing a field before it is initialized. When one or more types in the DAG define and override the same method, then method resolution rules need to be understood. This chapter explores these concepts. As we’ll see, they are closely related, governed by a concept called linearization.

Linearization of a Type Hierarchy

Because of single inheritance, if we ignored mixed-in traits, the inheritance hierarchy would be a simple linear relationship, one ancestor after another. When traits are considered, each of which may be a subtype of other traits and classes, the inheritance hierarchy forms a DAG.

The term linearization refers to the algorithm used to flatten this graph to determine the order of construction of the types in the graph, as well as the ordering of method invocations, including binding of super to invoke a supertype method. Think of the linearized traversal path flowing left to right with the supertypes to the left and the subtypes to the right. Construction follows this left-to-right ordering, meaning supertypes are constructed before subtypes. In contrast, when resolving which overloaded method to call in a hierarchy, the traversal goes right to left, where nearest super class definitions of the method take precedence over ...