Github user aljoscha commented on the issue:
https://github.com/apache/flink/pull/4616
The problem is that `RichSinkFunction` defines
```
public abstract void invoke(IN value) throws Exception;
```
even though it implements `SinkFunction`, which defines:
```
void invoke(IN value) throws Exception;
```
What this means is that code that is invoking these methods is different,
depending on the type of reference. For example, in
```
SinkFunction<String> interfaceSink = new SinkFunction<String>() {
@Override
public void invoke(String value) throws Exception {
}
};
SinkFunction<String> interfaceSinkToRichSink = new
RichSinkFunction<String>() {
@Override
public void invoke(String value) throws Exception {
}
};
RichSinkFunction<String> richSink = new RichSinkFunction<String>() {
@Override
public void invoke(String value) throws Exception {
}
};
interfaceSink.invoke("hello");
interfaceSinkToRichSink.invoke("hello");
richSink.invoke("hello");
```
the first two calls are `invokeinterface` while the last call is using
`invokevirtual`.
I don't think that people are using the sink interfaces for directly
calling methods on them but this is the reason the binary compatibility checker
is complaining.
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