@SuppressWarnings("unchecked")
        public KTableKTableJoinNode<K, V1, V2, VR> build() {
            return new KTableKTableJoinNode<>(
                nodeName,
                joinThisProcessorParameters,
                joinOtherProcessorParameters,
                new ProcessorParameters<>(
                    KTableKTableJoinMerger.of(
                        (KTableProcessorSupplier<K, V1, K, VR>) joinThisProcessorParameters.processorSupplier(),
                        (KTableProcessorSupplier<K, V2, K, VR>) joinOtherProcessorParameters.processorSupplier(),
                        queryableStoreName),
                    nodeName),
                thisJoinSide,
                otherJoinSide,
                keySerde,
                valueSerde,
                joinThisStoreNames,
                joinOtherStoreNames,
                storeFactory
            );
        }
The supplier which provides processor with KTable-KTable join merge functionality.
streams/src/main/java/org/apache/kafka/streams/kstream/internals/graph/KTableKTableJoinNode.java