What happens if I forget the 'override' keyword?

← Override Mechanics · Ref: Q571

If your method has the same name and signature as a parent method but you omit the 'override' keyword, the compiler reports E05120. This prevents accidental method shadowing.

THE ERROR: E05120

E05120 fires when a method matches a parent method exactly but lacks the 'override' keyword. The compiler refuses to silently shadow the parent behavior.

FIX: ADD OVERRIDE

Simply add 'override' before the method name. Before (error):

  defines class
    Parent as open
      process()
        <- rtn as String: "parent"
    Child extends Parent
      process()    // ERROR E05120
        <- rtn as String: "child"

After (correct):

  defines class
    Parent as open
      process()
        <- rtn as String: "parent"
    Child extends Parent
      override process()
        <- rtn as String: "child"

ALTERNATIVE: RENAME

If you intended a new method (not an override), rename it to avoid matching the parent signature.

WHY EXPLICIT OVERRIDE MATTERS

In large codebases, methods can accidentally match parent signatures after refactoring. Without mandatory override, this creates subtle bugs where the child silently replaces parent behavior. EK9 catches this at compile time.

See Q570 for override basics. See Q572 for the opposite: claiming override when nothing matches. See Q105 for method dispatch.

Example

defines module qa.override.missingkeyword

  defines class

    Greeter as open
      greet() as pure
        -> recipient as String
        <- rtn as String: "Hello, " + recipient

      farewell() as pure
        -> recipient as String
        <- rtn as String: "Goodbye, " + recipient

      default operator ?

    //Correct: override keyword is present
    FormalGreeter extends Greeter
      override greet() as pure
        -> recipient as String
        <- rtn as String: "Good day, " + recipient

      override farewell() as pure
        -> recipient as String
        <- rtn as String: "Farewell, " + recipient

      //New method: no override needed (different name)
      bow() as pure
        -> recipient as String
        <- rtn as String: "Bowing to " + recipient

      default operator ?

  defines program

    MissingOverrideDemo()
      stdout <- Stdout()

      recipientName <- "Alice"
      greeter <- FormalGreeter()
      stdout.println(greeter.greet(recipientName))
      stdout.println(greeter.farewell(recipientName))
      stdout.println(greeter.bow(recipientName))

Common mistakes

E05120 — When a method matches a parent method signature, omitting override causes a shadowing error. The compiler requires explicit intent. See ek9 -h E05120 for details.

Incorrect:

greet() as pure

Correct:

override greet() as pure

E05120 — farewell() has the same name and signature as Greeter.farewell(), so the override keyword is required; omitting it is E05120. See ek9 -h E05120 for details.

Incorrect:

farewell() as pure

Correct:

override farewell() as pure

Coming from another language?

Java: @Override is optional, shadowing is silent without it. Python: no override mechanism. Kotlin: override is mandatory (same as EK9). C#: override keyword required for virtual methods. EK9: override is mandatory, E05120 reported if missing.

Keywords: shadow, open, virtual, explicit, E05120, keyword, parent, abstract, inherit, override, match, missing