Github user wuchong commented on a diff in the pull request:
https://github.com/apache/flink/pull/3176#discussion_r100470762
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/functions/UserDefinedFunction.scala
---
@@ -24,4 +24,19 @@ package org.apache.flink.table.functions
* User-defined functions must have a default constructor and must be
instantiable during runtime.
*/
trait UserDefinedFunction {
--- End diff --
Copy from the blog you link:
>To summarize: If youâre writing a Scala API that will be used by Java
clients, donât expose traits that have implemented behavior in your public
API. If you have traits like that, wrap them in a class for your Java consumers.
The wrapper class mentioned in the blog is a concrete class defined in
Scala and not an abstract class (like `ScalarFunction`). And if we extend this
class in Java, the same error will be thrown. I think define
`UserDefinedFunction` as abstract can easily fix this problem and no side
effects.
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