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We will hint to the DSL users via Java docs that they have now have the option to distinguish between the following scenarios within a KTable-KTable foreign- key-left-join operator:
"null-key" v.s. "not-null-key but null-value" by passing a 'ValueJoinerWithKey' instead of 'ValueJoiner' to the left join operator.
The remark will be made in the Java docs of all left join methods with a 'ValueJoiner' as a parameter.
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