MINOR: reduce() javadocs: clarify position of arguments

Author: Michael G. Noll <michael@confluent.io>

Reviewers: Matthias J. Sax <matthias@confluent.io>, Eno Thereska <eno.thereska@gmail.com>, Damian Guy <damian.guy@gmail.com>

Closes #2651 from miguno/trunk-reduce-javadocs

streams/src/main/java/org/apache/kafka/streams/kstream/KGroupedStream.java

@@ -409,7 +409,16 @@ public interface KGroupedStream<K, V> {
      * Furthermore, updates to the store are sent downstream into a {@link KTable} changelog stream.
      * <p>
      * The specified {@link Reducer} is applied for each input record and computes a new aggregate using the current
-     * aggregate and the record's value.
+     * aggregate (first argument) and the record's value (second argument):
+     * <pre>{@code
+     * // At the example of a Reducer<Long>
+     * new Reducer<Long>() {
+     *   @Override
+     *   public Long apply(Long aggValue, Long currValue) {
+     *     return aggValue + currValue;
+     *   }
+     * }</pre>
+     * <p>
      * If there is no current aggregate the {@link Reducer} is not applied and the new aggregate will be the record's
      * value as-is.
      * Thus, {@code reduce(Reducer, String)} can be used to compute aggregate functions like sum, min, or max.
@@ -461,7 +470,16 @@ public interface KGroupedStream<K, V> {
      * Furthermore, updates to the store are sent downstream into a {@link KTable} changelog stream.
      * <p>
      * The specified {@link Reducer} is applied for each input record and computes a new aggregate using the current
-     * aggregate and the record's value.
+     * aggregate (first argument) and the record's value (second argument):
+     * <pre>{@code
+     * // At the example of a Reducer<Long>
+     * new Reducer<Long>() {
+     *   @Override
+     *   public Long apply(Long aggValue, Long currValue) {
+     *     return aggValue + currValue;
+     *   }
+     * }</pre>
+     * <p>
      * If there is no current aggregate the {@link Reducer} is not applied and the new aggregate will be the record's
      * value as-is.
      * Thus, {@code reduce(Reducer, StateStoreSupplier)} can be used to compute aggregate functions like sum, min, or
@@ -509,7 +527,16 @@ public interface KGroupedStream<K, V> {
      * "windowed" implies that the {@link KTable} key is a combined key of the original record key and a window ID.
      * <p>
      * The specified {@link Reducer} is applied for each input record and computes a new aggregate using the current
-     * aggregate and the record's value.
+     * aggregate (first argument) and the record's value (second argument):
+     * <pre>{@code
+     * // At the example of a Reducer<Long>
+     * new Reducer<Long>() {
+     *   @Override
+     *   public Long apply(Long aggValue, Long currValue) {
+     *     return aggValue + currValue;
+     *   }
+     * }</pre>
+     * <p>
      * If there is no current aggregate the {@link Reducer} is not applied and the new aggregate will be the record's
      * value as-is.
      * Thus, {@code reduce(Reducer, Windows, String)} can be used to compute aggregate functions like sum, min, or max.
@@ -610,7 +637,16 @@ public interface KGroupedStream<K, V> {
      * "windowed" implies that the {@link KTable} key is a combined key of the original record key and a window ID.
      * <p>
      * The specified {@link Reducer} is applied for each input record and computes a new aggregate using the current
-     * aggregate and the record's value.
+     * aggregate (first argument) and the record's value (second argument):
+     * <pre>{@code
+     * // At the example of a Reducer<Long>
+     * new Reducer<Long>() {
+     *   @Override
+     *   public Long apply(Long aggValue, Long currValue) {
+     *     return aggValue + currValue;
+     *   }
+     * }</pre>
+     * <p>
      * If there is no current aggregate the {@link Reducer} is not applied and the new aggregate will be the record's
      * value as-is.
      * Thus, {@code reduce(Reducer, Windows, StateStoreSupplier)} can be used to compute aggregate functions like sum,
@@ -660,7 +696,16 @@ public interface KGroupedStream<K, V> {
      * "windowed" implies that the {@link KTable} key is a combined key of the original record key and a window ID.
      * <p>
      * The specified {@link Reducer} is applied for each input record and computes a new aggregate using the current
-     * aggregate and the record's value.
+     * aggregate (first argument) and the record's value (second argument):
+     * <pre>{@code
+     * // At the example of a Reducer<Long>
+     * new Reducer<Long>() {
+     *   @Override
+     *   public Long apply(Long aggValue, Long currValue) {
+     *     return aggValue + currValue;
+     *   }
+     * }</pre>
+     * <p>
      * If there is no current aggregate the {@link Reducer} is not applied and the new aggregate will be the record's
      * value as-is.
      * Thus, {@code reduce(Reducer, SessionWindows, String)} can be used to compute aggregate functions like sum, min,
@@ -749,7 +794,16 @@ public interface KGroupedStream<K, V> {
      * "windowed" implies that the {@link KTable} key is a combined key of the original record key and a window ID.
      * <p>
      * The specified {@link Reducer} is applied for each input record and computes a new aggregate using the current
-     * aggregate and the record's value.
+     * aggregate (first argument) and the record's value (second argument):
+     * <pre>{@code
+     * // At the example of a Reducer<Long>
+     * new Reducer<Long>() {
+     *   @Override
+     *   public Long apply(Long aggValue, Long currValue) {
+     *     return aggValue + currValue;
+     *   }
+     * }</pre>
+     * <p>
      * If there is no current aggregate the {@link Reducer} is not applied and the new aggregate will be the record's
      * value as-is.
      * Thus, {@code reduce(Reducer, SessionWindows, StateStoreSupplier)} can be used to compute aggregate functions like

streams/src/main/java/org/apache/kafka/streams/kstream/KGroupedTable.java

@@ -163,10 +163,11 @@ public interface KGroupedTable<K, V> {
      * <p>
      * Each update to the original {@link KTable} results in a two step update of the result {@link KTable}.
      * The specified {@link Reducer adder} is applied for each update record and computes a new aggregate using the
-     * current aggregate and the record's value by adding the new record to the aggregate.
+     * current aggregate (first argument) and the record's value (second argument) by adding the new record to the
+     * aggregate.
      * The specified {@link Reducer substractor} is applied for each "replaced" record of the original {@link KTable}
-     * and computes a new aggregate using the current aggregate and the record's value by "removing" the "replaced"
-     * record from the aggregate.
+     * and computes a new aggregate using the current aggregate (first argument) and the record's value (second
+     * argument) by "removing" the "replaced" record from the aggregate.
      * If there is no current aggregate the {@link Reducer} is not applied and the new aggregate will be the record's
      * value as-is.
      * Thus, {@code reduce(Reducer, Reducer, String)} can be used to compute aggregate functions like sum.
@@ -290,10 +291,11 @@ public interface KGroupedTable<K, V> {
      * <p>
      * Each update to the original {@link KTable} results in a two step update of the result {@link KTable}.
      * The specified {@link Reducer adder} is applied for each update record and computes a new aggregate using the
-     * current aggregate and the record's value by adding the new record to the aggregate.
+     * current aggregate (first argument) and the record's value (second argument) by adding the new record to the
+     * aggregate.
      * The specified {@link Reducer substractor} is applied for each "replaced" record of the original {@link KTable}
-     * and computes a new aggregate using the current aggregate and the record's value by "removing" the "replaced"
-     * record from the aggregate.
+     * and computes a new aggregate using the current aggregate (first argument) and the record's value (second
+     * argument) by "removing" the "replaced" record from the aggregate.
      * If there is no current aggregate the {@link Reducer} is not applied and the new aggregate will be the record's
      * value as-is.
      * Thus, {@code reduce(Reducer, Reducer, String)} can be used to compute aggregate functions like sum.