public static <K, V1, V2, VR> KTableKTableJoinNodeBuilder<K, V1, V2, VR> kTableKTableJoinNodeBuilder() {
        return new KTableKTableJoinNodeBuilder<>();
    }
The supplier which provides processor with KTable-KTable join merge functionality.
streams/src/main/java/org/apache/kafka/streams/kstream/internals/graph/KTableKTableJoinNode.java