E02010: Property/Field duplicated

Category: E02xxx Symbol definition · Reported in phase: FULL_RESOLUTION · Classification: DUPLICATE_PROPERTY_FIELD · All error codes

What went wrong

A property/field with this name already exists in the type hierarchy. Child types cannot redeclare properties that exist in parent types - records, classes, components and dynamic classes alike, anywhere up the super chain, whatever the type.

Worked examples

Compilable examples demonstrating this error and the correct fix:

ek9 -q 819 (Why does EK9 reject duplicate property fields in records?)
ek9 -q 829 (Why does EK9 reject duplicate properties in records with ...)

How to fix it

Set the inherited property instead of redeclaring it (in the constructor, or through the parent's constructor or methods)
Use a different property name if it really is different state
If different type needed, reconsider your type hierarchy design

Rationale

Properties are inherited in EK9. Redeclaring creates ambiguity:

Which 'prop3' should be used when accessed?
JSON serialization would have duplicate keys
Assignment becomes unpredictable

Class and component properties are private, but a redeclared one is still a SECOND field: the parent's methods read the parent's copy and the child's methods read the child's copy, so one piece of state silently splits in two. EK9 rejects it rather than allow that.

Common causes

Child class or component declares property with same name as parent
Redeclaring a property to give it a different initial value
Record extends another and redeclares a field
Forgetting that properties are inherited

Examples

Every example is compiled by the EK9 compiler: the first fails exactly as captioned, the others fix it.

Fails with E02010: Property/Field duplicated

#!ek9
defines module error.e02010.bad

  defines record
    RBase1 as open
      prop1 <- String()
      prop3 <- Date()

      default operator ?

    RExtension1 extends RBase1
      prop3 <- String()

      default operator ?

    RExtension2 extends RBase1
      prop1 <- String()
      prop3 <- Time()

      default operator ?

Fixed: An extension adds properties of its own; it never redeclares an inherited one

#!ek9
defines module error.e02010.good

  defines record

    RBase1 as open
      prop1 <- String()
      prop3 <- Date()
      default operator ?

    RExtension1 extends RBase1
      prop4 <- String()
      default operator ?

    RExtension2 extends RBase1
      prop5 <- Time()
      default operator ?

Questions and answers

These Q&A entries show E02010 and its fix: