THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
...
Some non-API types (with @Internalannotation, or without any annotation) are used for arguments / return-values of APIs. E.g., the @InternaltypeStreamRecordis used for the @PublicEvolvingAPI `StreamOperator#setKeyContextElement1/setKeyContextElement2`.
User programs and runtime codes are depending on the same concrete classes, which means methods that are intended to only be used for runtime codes can also be accessed by user programs, despite annotated as @Internalor not. E.g., `DataStream#getTransformation`.
Many APIs are designed to let users extends an abstract / concrete class. This is problematic because user classes can access the internals of the super class and override its behaviors, leading to unexpectable problems. EMany APIs are designed to let users extends an abstract / concrete class. This is problematic because user classes can access the internals of the super class and override its behaviors, leading to unexpectable problems. E.g., AbstractStreamOperator.
StreamOperator, which is currently a @PublicEvolvingAPI, is closely coupled with many runtime internal concepts: checkpoints / snapshots, key context, containing task, etc. These concepts are unnecessarily exposed to users.
...