Extend a BaseService component and add a version field to the child.

Mark parent 'as open' and extend with child:

  BaseService as open
    serviceName <- "Base"
    default operator

  ExtendedService extends BaseService
    version <- 1
    default operator

Child inherits parent fields. 'default operator' covers both parent and child fields.

See Q1105 for basic components. See Q1080 for class inheritance with :=: copy.

Example

defines module qa.di.componentinheritance

  defines component

    BaseService as open
      serviceName <- "BaseService"

      default operator

  defines component

    ExtendedService extends BaseService
      version <- 1

      default operator

  defines program

    ComponentInheritanceDemo()
      stdout <- Stdout()

      base <- BaseService()
      stdout.println(`Base: ${base}`)

      ext <- ExtendedService()
      stdout.println(`Extended: ${ext}`)
      stdout.println(`Extended set?: ${ext?}`)

Common mistakes

E05030 — Components are closed by default. Add 'as open' to allow extension.

Incorrect:

    BaseService

Correct:

    BaseService as open

Other ways to ask this

I have a base component and need to create a specialised version with extra fields
In Spring I'd extend a @Service class. Write the EK9 component inheritance
Given a BaseService, create an ExtendedService that adds version tracking
Build a component hierarchy with parent and child fields

Coming from another language?

Spring: extend a @Component class. Guice: bind subclass to parent interface. EK9: component extends with 'as open' on parent.

Keywords: inheritance, open, extends, parent, child, component