code: public AvroMultiRuntimeDefinition build(){
      Validate.isTrue(this.runtimes.size() == 1 || !StringUtils.isEmpty(this.defaultRuntime), "Default runtime " +
              "should be set if more than a single runtime provided");
  
      if(StringUtils.isEmpty(this.defaultRuntime)){
        // we have single runtime configured, take its name as a default
        this.defaultRuntime = this.runtimes.keySet().iterator().next();
      }
  
      Validate.isTrue(this.runtimes.containsKey(this.defaultRuntime), "Default runtime should be configured");
      return new AvroMultiRuntimeDefinition(defaultRuntime, new ArrayList<>(this.runtimes.values()));
    }
docs: Builds multi runtime definition.
  @return The populated definition object