zero_coding :
I have the following code:
source
.mapValues(value -> value + " Stream it!!!")
.print(Printed.toSysOut());
as you can see, mapValues expects a lambda expression.
Now, I am using Java library but the application is written in Scala. How to pass Scala lambda to Java code?
I tried the following:
source
.mapValues(value => value + "hello")
.print(Printed.toSysOut)
But the compiler complains:
[error] (x$1: org.apache.kafka.streams.kstream.Printed[String,?0(in value x$1)])Unit <and>
[error] (x$1: org.apache.kafka.streams.kstream.KeyValueMapper[_ >: String, _ >: ?0(in value x$1), String])Unit <and>
[error] (x$1: String)Unit
[error] cannot be applied to (org.apache.kafka.streams.kstream.Printed[Nothing,Nothing])
[error] .print(Printed.toSysOut)
[error] ^
[error] two errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 2 s, completed Nov 19, 2017 7:53:44 PM
Rich Dougherty :
The error message lists the types of arguments that print supports. One of them is:
org.apache.kafka.streams.kstream.Printed[String,?0(in value x$1)]
From the error message you can see that you're providing Printed.toSysOut with a type of:
org.apache.kafka.streams.kstream.Printed[Nothing,Nothing]
According to the Kafka 1 javadoc (Printed was not present in Kafka 1.1), toSysOut is defined as:
public static <K,V> Printed<K,V> toSysOut()
So the answer problem is that Scala is inferring K and V with types of Nothing. You need to provide the types explicitly.
The following will probably work:
source
.mapValues[String](value -> value + " Stream it!!!")
.print(Printed.toSysOut[String,String])
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