How to set result value Serdes Class in Kafka stream join

[sy.pan]

Hi, all:

Recently I have read kafka streams join 
document(https://docs.confluent.io/current/streams/developer-guide.html#kafka-streams-dsl
 
<https://docs.confluent.io/current/streams/developer-guide.html#kafka-streams-dsl>).
  The sample code is pasted below:

import java.util.concurrent.TimeUnit;
KStream<String, Long> left = ...;
KStream<String, Double> right = ...;
// Java 7 example
KStream<String, String> joined = left.join(right,
    new ValueJoiner<Long, Double, String>() {
      @Override
      public String apply(Long leftValue, Double rightValue) {
        return "left=" + leftValue + ", right=" + rightValue;
      }
    },
    JoinWindows.of(TimeUnit.MINUTES.toMillis(5)),
    Serdes.String(), /* key */
    Serdes.Long(),   /* left value */
    Serdes.Double()  /* right value */
  );

so the question is :

1) which parameter is used for setting result value(returned by ValueJoiner) 
Serdes class ?
    the sample code only set key , left value and right value Serdes class.

2) if ValueJoiner return customer value type,  how to set the result value 
Serdes class ?