From 531b389da8424aa2c594e3d2e186221fbb914d08 Mon Sep 17 00:00:00 2001
From: akarnokd <akarnokd@gmail.com>
Date: Thu, 19 Mar 2026 12:50:35 +0100
Subject: [PATCH 1/3] 4.x: Introduce FlatMapConfig & remove some overload bloat

Also found a Checkstyle plugin bug:
https://github.com/checkstyle/checkstyle/issues/19342
---
 config/checkstyle/suppressions.xml            |   4 +
 gradle/wrapper/gradle-wrapper.properties      |   2 +-
 gradlew                                       |   2 +-
 .../FlowableFlatMapCompletableAsyncPerf.java  |   3 +-
 .../FlowableFlatMapCompletableSyncPerf.java   |   3 +-
 .../rxjava4/parallel/ParallelPerf.java        |   3 +-
 .../io/reactivex/rxjava4/core/Flowable.java   | 172 +++---------------
 .../rxjava4/core/config/FlatMapConfig.java    |  74 ++++++++
 .../rxjava4/core/config/GenericConfig.java    |  74 ++++++++
 .../rxjava4/core/docs/FlowableDocBasic.java   |   1 +
 src/main/module/module-info.java              |   2 +
 .../flowable/FlowableFlatMapTest.java         |  40 ++--
 .../flowable/FlowableGroupByTest.java         |  25 +--
 .../operators/flowable/FlowableMergeTest.java |   5 +-
 .../operators/flowable/FlowableRetryTest.java |   3 +-
 .../FlowableWindowWithFlowableTest.java       |   3 +-
 .../flowable/FlowableWindowWithTimeTest.java  |   3 +-
 .../ParamValidationCheckerTest.java           |   6 +-
 18 files changed, 237 insertions(+), 188 deletions(-)
 create mode 100644 src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java
 create mode 100644 src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java

diff --git a/config/checkstyle/suppressions.xml b/config/checkstyle/suppressions.xml
index f4675fd64c..0d28559996 100644
--- a/config/checkstyle/suppressions.xml
+++ b/config/checkstyle/suppressions.xml
@@ -12,4 +12,8 @@
   <suppress checks="MissingJavadocMethod|JavadocMethod" files=".*Test\.java$"/>
   <suppress checks="MissingJavadocMethod|JavadocMethod" files=".*Perf\.java$"/>
 
+  <!-- FIXME Checkstyle Bug: https://github.com/checkstyle/checkstyle/issues/19342 -->
+  <suppress checks=".*" files=".*[\\/]FlatMapConfig\.java$"/>
+  <suppress checks=".*" files=".*[\\/]GenericConfig\.java$"/>
+  
 </suppressions>
diff --git a/gradle/wrapper/gradle-wrapper.properties b/gradle/wrapper/gradle-wrapper.properties
index dbc3ce4a04..c61a118f7d 100644
--- a/gradle/wrapper/gradle-wrapper.properties
+++ b/gradle/wrapper/gradle-wrapper.properties
@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-9.4.0-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.4.1-bin.zip
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME
diff --git a/gradlew b/gradlew
index 0262dcbd52..739907dfd1 100755
--- a/gradlew
+++ b/gradlew
@@ -57,7 +57,7 @@
 #       Darwin, MinGW, and NonStop.
 #
 #   (3) This script is generated from the Groovy template
-#       https://github.com/gradle/gradle/blob/b631911858264c0b6e4d6603d677ff5218766cee/platforms/jvm/plugins-application/src/main/resources/org/gradle/api/internal/plugins/unixStartScript.txt
+#       https://github.com/gradle/gradle/blob/2d6327017519d23b96af35865dc997fcb544fb40/platforms/jvm/plugins-application/src/main/resources/org/gradle/api/internal/plugins/unixStartScript.txt
 #       within the Gradle project.
 #
 #       You can find Gradle at https://github.com/gradle/gradle/.
diff --git a/src/jmh/java/io/reactivex/rxjava4/core/FlowableFlatMapCompletableAsyncPerf.java b/src/jmh/java/io/reactivex/rxjava4/core/FlowableFlatMapCompletableAsyncPerf.java
index 548927d771..07efe9ca9c 100644
--- a/src/jmh/java/io/reactivex/rxjava4/core/FlowableFlatMapCompletableAsyncPerf.java
+++ b/src/jmh/java/io/reactivex/rxjava4/core/FlowableFlatMapCompletableAsyncPerf.java
@@ -19,6 +19,7 @@
 import org.openjdk.jmh.annotations.*;
 import org.openjdk.jmh.infra.Blackhole;
 
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.functions.Action;
 import io.reactivex.rxjava4.internal.functions.Functions;
 import io.reactivex.rxjava4.schedulers.Schedulers;
@@ -59,7 +60,7 @@ public void setup() {
                 .flatMapCompletable(Functions.justFunction(Completable.fromAction(this).subscribeOn(Schedulers.computation())), false, maxConcurrency);
 
         flatMap = Flowable.fromArray(array)
-                .flatMap(Functions.justFunction(Completable.fromAction(this).subscribeOn(Schedulers.computation()).toFlowable()), false, maxConcurrency);
+                .flatMap(Functions.justFunction(Completable.fromAction(this).subscribeOn(Schedulers.computation()).toFlowable()), new FlatMapConfig(false, maxConcurrency));
     }
 
 //    @Benchmark
diff --git a/src/jmh/java/io/reactivex/rxjava4/core/FlowableFlatMapCompletableSyncPerf.java b/src/jmh/java/io/reactivex/rxjava4/core/FlowableFlatMapCompletableSyncPerf.java
index 4f2460989a..580efd9ad4 100644
--- a/src/jmh/java/io/reactivex/rxjava4/core/FlowableFlatMapCompletableSyncPerf.java
+++ b/src/jmh/java/io/reactivex/rxjava4/core/FlowableFlatMapCompletableSyncPerf.java
@@ -19,6 +19,7 @@
 import org.openjdk.jmh.annotations.*;
 import org.openjdk.jmh.infra.Blackhole;
 
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.internal.functions.Functions;
 
 @SuppressWarnings("exports")
@@ -49,7 +50,7 @@ public void setup() {
                 .flatMapCompletable(Functions.justFunction(Completable.complete()), false, maxConcurrency);
 
         flatMap = Flowable.fromArray(array)
-                .flatMap(Functions.justFunction(Completable.complete().toFlowable()), false, maxConcurrency);
+                .flatMap(Functions.justFunction(Completable.complete().toFlowable()), new FlatMapConfig(false, maxConcurrency));
     }
 
     @Benchmark
diff --git a/src/jmh/java/io/reactivex/rxjava4/parallel/ParallelPerf.java b/src/jmh/java/io/reactivex/rxjava4/parallel/ParallelPerf.java
index 3334aa5247..b4f9f747c7 100644
--- a/src/jmh/java/io/reactivex/rxjava4/parallel/ParallelPerf.java
+++ b/src/jmh/java/io/reactivex/rxjava4/parallel/ParallelPerf.java
@@ -21,6 +21,7 @@
 import static java.util.concurrent.Flow.*;
 
 import io.reactivex.rxjava4.core.*;
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.flowables.GroupedFlowable;
 import io.reactivex.rxjava4.functions.Function;
 import io.reactivex.rxjava4.schedulers.Schedulers;
@@ -71,7 +72,7 @@ public Publisher<Integer> apply(Integer v) {
                 return Flowable.just(v).subscribeOn(Schedulers.computation())
                         .map(ParallelPerf.this);
             }
-        }, cpu);
+        }, new FlatMapConfig(cpu));
 
         groupBy = source.groupBy(new Function<Integer, Integer>() {
             int i;
diff --git a/src/main/java/io/reactivex/rxjava4/core/Flowable.java b/src/main/java/io/reactivex/rxjava4/core/Flowable.java
index 726dddd5e5..b71cbf7545 100644
--- a/src/main/java/io/reactivex/rxjava4/core/Flowable.java
+++ b/src/main/java/io/reactivex/rxjava4/core/Flowable.java
@@ -19,6 +19,7 @@
 import java.util.stream.*;
 
 import io.reactivex.rxjava4.annotations.*;
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.core.docs.FlowableDocBasic;
 import io.reactivex.rxjava4.disposables.*;
 import io.reactivex.rxjava4.exceptions.*;
@@ -3498,7 +3499,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SchedulerSupport(SchedulerSupport.NONE)
     @NonNull
     public static <@NonNull T> Flowable<T> merge(@NonNull Iterable<@NonNull ? extends Publisher<? extends T>> sources, int maxConcurrency, int bufferSize) {
-        return fromIterable(sources).flatMap((Function)Functions.identity(), false, maxConcurrency, bufferSize);
+        return fromIterable(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(false, maxConcurrency, bufferSize));
     }
 
     /**
@@ -3551,7 +3552,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SafeVarargs
     @NonNull
     public static <@NonNull T> Flowable<T> mergeArray(int maxConcurrency, int bufferSize, @NonNull Publisher<? extends T>... sources) {
-        return fromArray(sources).flatMap((Function)Functions.identity(), false, maxConcurrency, bufferSize);
+        return fromArray(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(false, maxConcurrency, bufferSize));
     }
 
     /**
@@ -3646,7 +3647,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SchedulerSupport(SchedulerSupport.NONE)
     @NonNull
     public static <@NonNull T> Flowable<T> merge(@NonNull Iterable<@NonNull ? extends Publisher<? extends T>> sources, int maxConcurrency) {
-        return fromIterable(sources).flatMap((Function)Functions.identity(), maxConcurrency);
+        return fromIterable(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(maxConcurrency));
     }
 
     /**
@@ -3744,7 +3745,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SchedulerSupport(SchedulerSupport.NONE)
     @NonNull
     public static <@NonNull T> Flowable<T> merge(@NonNull Publisher<@NonNull ? extends Publisher<? extends T>> sources, int maxConcurrency) {
-        return fromPublisher(sources).flatMap((Function)Functions.identity(), maxConcurrency);
+        return fromPublisher(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(maxConcurrency));
     }
 
     /**
@@ -3790,7 +3791,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SafeVarargs
     @NonNull
     public static <@NonNull T> Flowable<T> mergeArray(@NonNull Publisher<? extends T>... sources) {
-        return fromArray(sources).flatMap((Function)Functions.identity(), sources.length);
+        return fromArray(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(sources.length));
     }
 
     /**
@@ -3839,7 +3840,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     public static <@NonNull T> Flowable<T> merge(@NonNull Publisher<? extends T> source1, @NonNull Publisher<? extends T> source2) {
         Objects.requireNonNull(source1, "source1 is null");
         Objects.requireNonNull(source2, "source2 is null");
-        return fromArray(source1, source2).flatMap((Function)Functions.identity(), false, 2);
+        return fromArray(source1, source2).flatMap((Function)Functions.identity(), new FlatMapConfig(false, 2));
     }
 
     /**
@@ -3891,7 +3892,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
         Objects.requireNonNull(source1, "source1 is null");
         Objects.requireNonNull(source2, "source2 is null");
         Objects.requireNonNull(source3, "source3 is null");
-        return fromArray(source1, source2, source3).flatMap((Function)Functions.identity(), false, 3);
+        return fromArray(source1, source2, source3).flatMap((Function)Functions.identity(), new FlatMapConfig(false, 3));
     }
 
     /**
@@ -3948,7 +3949,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
         Objects.requireNonNull(source2, "source2 is null");
         Objects.requireNonNull(source3, "source3 is null");
         Objects.requireNonNull(source4, "source4 is null");
-        return fromArray(source1, source2, source3, source4).flatMap((Function)Functions.identity(), false, 4);
+        return fromArray(source1, source2, source3, source4).flatMap((Function)Functions.identity(), new FlatMapConfig(false, 4));
     }
 
     /**
@@ -3985,7 +3986,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SchedulerSupport(SchedulerSupport.NONE)
     @NonNull
     public static <@NonNull T> Flowable<T> mergeDelayError(@NonNull Iterable<@NonNull ? extends Publisher<? extends T>> sources) {
-        return fromIterable(sources).flatMap((Function)Functions.identity(), true);
+        return fromIterable(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(true));
     }
 
     /**
@@ -4027,7 +4028,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SchedulerSupport(SchedulerSupport.NONE)
     @NonNull
     public static <@NonNull T> Flowable<T> mergeDelayError(@NonNull Iterable<@NonNull ? extends Publisher<? extends T>> sources, int maxConcurrency, int bufferSize) {
-        return fromIterable(sources).flatMap((Function)Functions.identity(), true, maxConcurrency, bufferSize);
+        return fromIterable(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(true, maxConcurrency, bufferSize));
     }
 
     /**
@@ -4070,7 +4071,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SafeVarargs
     @NonNull
     public static <@NonNull T> Flowable<T> mergeArrayDelayError(int maxConcurrency, int bufferSize, @NonNull Publisher<? extends T>... sources) {
-        return fromArray(sources).flatMap((Function)Functions.identity(), true, maxConcurrency, bufferSize);
+        return fromArray(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(true, maxConcurrency, bufferSize));
     }
 
     /**
@@ -4110,7 +4111,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SchedulerSupport(SchedulerSupport.NONE)
     @NonNull
     public static <@NonNull T> Flowable<T> mergeDelayError(@NonNull Iterable<@NonNull ? extends Publisher<? extends T>> sources, int maxConcurrency) {
-        return fromIterable(sources).flatMap((Function)Functions.identity(), true, maxConcurrency);
+        return fromIterable(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(true, maxConcurrency));
     }
 
     /**
@@ -4189,7 +4190,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SchedulerSupport(SchedulerSupport.NONE)
     @NonNull
     public static <@NonNull T> Flowable<T> mergeDelayError(@NonNull Publisher<@NonNull ? extends Publisher<? extends T>> sources, int maxConcurrency) {
-        return fromPublisher(sources).flatMap((Function)Functions.identity(), true, maxConcurrency);
+        return fromPublisher(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(true, maxConcurrency));
     }
 
     /**
@@ -4227,7 +4228,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     @SafeVarargs
     @NonNull
     public static <@NonNull T> Flowable<T> mergeArrayDelayError(@NonNull Publisher<? extends T>... sources) {
-        return fromArray(sources).flatMap((Function)Functions.identity(), true, sources.length);
+        return fromArray(sources).flatMap((Function)Functions.identity(), new FlatMapConfig(true, sources.length));
     }
 
     /**
@@ -4268,7 +4269,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
     public static <@NonNull T> Flowable<T> mergeDelayError(@NonNull Publisher<? extends T> source1, @NonNull Publisher<? extends T> source2) {
         Objects.requireNonNull(source1, "source1 is null");
         Objects.requireNonNull(source2, "source2 is null");
-        return fromArray(source1, source2).flatMap((Function)Functions.identity(), true, 2);
+        return fromArray(source1, source2).flatMap((Function)Functions.identity(), new FlatMapConfig(true, 2));
     }
 
     /**
@@ -4313,7 +4314,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
         Objects.requireNonNull(source1, "source1 is null");
         Objects.requireNonNull(source2, "source2 is null");
         Objects.requireNonNull(source3, "source3 is null");
-        return fromArray(source1, source2, source3).flatMap((Function)Functions.identity(), true, 3);
+        return fromArray(source1, source2, source3).flatMap((Function)Functions.identity(), new FlatMapConfig(true, 3));
     }
 
     /**
@@ -4363,7 +4364,7 @@ public static Flowable<Long> intervalRange(long start, long count, long initialD
         Objects.requireNonNull(source2, "source2 is null");
         Objects.requireNonNull(source3, "source3 is null");
         Objects.requireNonNull(source4, "source4 is null");
-        return fromArray(source1, source2, source3, source4).flatMap((Function)Functions.identity(), true, 4);
+        return fromArray(source1, source2, source3, source4).flatMap((Function)Functions.identity(), new FlatMapConfig(true, 4));
     }
 
     /**
@@ -10261,42 +10262,7 @@ public final Single<T> firstOrError() {
     @SchedulerSupport(SchedulerSupport.NONE)
     @NonNull
     public final <@NonNull R> Flowable<R> flatMap(@NonNull Function<? super T, @NonNull ? extends Publisher<? extends R>> mapper) {
-        return flatMap(mapper, false, bufferSize(), bufferSize());
-    }
-
-    /**
-     * Returns a {@code Flowable} that emits items based on applying a function that you supply to each item emitted
-     * by the current {@code Flowable}, where that function returns a {@link Publisher}, and then merging those resulting
-     * {@code Publisher}s and emitting the results of this merger.
-     * <p>
-     * <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/flatMap.v3.png" alt="">
-     * <dl>
-     *  <dt><b>Backpressure:</b></dt>
-     *  <dd>The operator honors backpressure from downstream. The upstream {@code Flowable} is consumed
-     *  in a bounded manner (up to {@link #bufferSize()} outstanding request amount for items).
-     *  The inner {@code Publisher}s are expected to honor backpressure; if violated,
-     *  the operator <em>may</em> signal {@link MissingBackpressureException}.</dd>
-     *  <dt><b>Scheduler:</b></dt>
-     *  <dd>{@code flatMap} does not operate by default on a particular {@link Scheduler}.</dd>
-     * </dl>
-     *
-     * @param <R> the value type of the inner {@code Publisher}s and the output type
-     * @param mapper
-     *            a function that, when applied to an item emitted by the current {@code Flowable}, returns a
-     *            {@code Publisher}
-     * @param delayErrors
-     *            if {@code true}, exceptions from the current {@code Flowable} and all inner {@code Publisher}s are delayed until all of them terminate
-     *            if {@code false}, the first one signaling an exception will terminate the whole sequence immediately
-     * @return the new {@code Flowable} instance
-     * @throws NullPointerException if {@code mapper} is {@code null}
-     * @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>
-     */
-    @CheckReturnValue
-    @BackpressureSupport(BackpressureKind.FULL)
-    @SchedulerSupport(SchedulerSupport.NONE)
-    @NonNull
-    public final <@NonNull R> Flowable<R> flatMap(@NonNull Function<? super T, @NonNull ? extends Publisher<? extends R>> mapper, boolean delayErrors) {
-        return flatMap(mapper, delayErrors, bufferSize(), bufferSize());
+        return flatMap(mapper, new FlatMapConfig());
     }
 
     /**
@@ -10320,93 +10286,9 @@ public final Single<T> firstOrError() {
      * @param mapper
      *            a function that, when applied to an item emitted by the current {@code Flowable}, returns a
      *            {@code Publisher}
-     * @param maxConcurrency
-     *         the maximum number of {@code Publisher}s that may be subscribed to concurrently
+     * @param config the {@link FlatMapConfig} record to customize behavior
      * @return the new {@code Flowable} instance
-     * @throws NullPointerException if {@code mapper} is {@code null}
-     * @throws IllegalArgumentException if {@code maxConcurrency} is non-positive
-     * @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>
-     * @since 2.0
-     */
-    @CheckReturnValue
-    @BackpressureSupport(BackpressureKind.FULL)
-    @SchedulerSupport(SchedulerSupport.NONE)
-    @NonNull
-    public final <@NonNull R> Flowable<R> flatMap(@NonNull Function<? super T, @NonNull ? extends Publisher<? extends R>> mapper, int maxConcurrency) {
-        return flatMap(mapper, false, maxConcurrency, bufferSize());
-    }
-
-    /**
-     * Returns a {@code Flowable} that emits items based on applying a function that you supply to each item emitted
-     * by the current {@code Flowable}, where that function returns a {@link Publisher}, and then merging those resulting
-     * {@code Publisher}s and emitting the results of this merger, while limiting the maximum number of concurrent
-     * subscriptions to these {@code Publisher}s.
-     * <!-- <p> -->
-     * <!-- <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/flatMap.v3.png" alt=""> -->
-     * <dl>
-     *  <dt><b>Backpressure:</b></dt>
-     *  <dd>The operator honors backpressure from downstream. The upstream {@code Flowable} is consumed
-     *  in a bounded manner (up to {@code maxConcurrency} outstanding request amount for items).
-     *  The inner {@code Publisher}s are expected to honor backpressure; if violated,
-     *  the operator <em>may</em> signal {@link MissingBackpressureException}.</dd>
-     *  <dt><b>Scheduler:</b></dt>
-     *  <dd>{@code flatMap} does not operate by default on a particular {@link Scheduler}.</dd>
-     * </dl>
-     *
-     * @param <R> the value type of the inner {@code Publisher}s and the output type
-     * @param mapper
-     *            a function that, when applied to an item emitted by the current {@code Flowable}, returns a
-     *            {@code Publisher}
-     * @param maxConcurrency
-     *         the maximum number of {@code Publisher}s that may be subscribed to concurrently
-     * @param delayErrors
-     *            if {@code true}, exceptions from the current {@code Flowable} and all inner {@code Publisher}s are delayed until all of them terminate
-     *            if {@code false}, the first one signaling an exception will terminate the whole sequence immediately
-     * @return the new {@code Flowable} instance
-     * @throws NullPointerException if {@code mapper} is {@code null}
-     * @throws IllegalArgumentException if {@code maxConcurrency} is non-positive
-     * @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>
-     * @since 2.0
-     */
-    @CheckReturnValue
-    @BackpressureSupport(BackpressureKind.FULL)
-    @SchedulerSupport(SchedulerSupport.NONE)
-    @NonNull
-    public final <@NonNull R> Flowable<R> flatMap(@NonNull Function<? super T, @NonNull ? extends Publisher<? extends R>> mapper, boolean delayErrors, int maxConcurrency) {
-        return flatMap(mapper, delayErrors, maxConcurrency, bufferSize());
-    }
-
-    /**
-     * Returns a {@code Flowable} that emits items based on applying a function that you supply to each item emitted
-     * by the current {@code Flowable}, where that function returns a {@link Publisher}, and then merging those resulting
-     * {@code Publisher}s and emitting the results of this merger, while limiting the maximum number of concurrent
-     * subscriptions to these {@code Publisher}s.
-     * <!-- <p> -->
-     * <!-- <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/flatMap.v3.png" alt=""> -->
-     * <dl>
-     *  <dt><b>Backpressure:</b></dt>
-     *  <dd>The operator honors backpressure from downstream. The upstream {@code Flowable} is consumed
-     *  in a bounded manner (up to {@code maxConcurrency} outstanding request amount for items).
-     *  The inner {@code Publisher}s are expected to honor backpressure; if violated,
-     *  the operator <em>may</em> signal {@link MissingBackpressureException}.</dd>
-     *  <dt><b>Scheduler:</b></dt>
-     *  <dd>{@code flatMap} does not operate by default on a particular {@link Scheduler}.</dd>
-     * </dl>
-     *
-     * @param <R> the value type of the inner {@code Publisher}s and the output type
-     * @param mapper
-     *            a function that, when applied to an item emitted by the current {@code Flowable}, returns a
-     *            {@code Publisher}
-     * @param maxConcurrency
-     *         the maximum number of {@code Publisher}s that may be subscribed to concurrently
-     * @param delayErrors
-     *            if {@code true}, exceptions from the current {@code Flowable} and all inner {@code Publisher}s are delayed until all of them terminate
-     *            if {@code false}, the first one signaling an exception will terminate the whole sequence immediately
-     * @param bufferSize
-     *            the number of elements to prefetch from each inner {@code Publisher}
-     * @return the new {@code Flowable} instance
-     * @throws NullPointerException if {@code mapper} is {@code null}
-     * @throws IllegalArgumentException if {@code maxConcurrency} or {@code bufferSize} is non-positive
+     * @throws NullPointerException if {@code mapper} or {@code config} is {@code null}
      * @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>
      * @since 2.0
      */
@@ -10414,11 +10296,11 @@ public final Single<T> firstOrError() {
     @NonNull
     @BackpressureSupport(BackpressureKind.FULL)
     @SchedulerSupport(SchedulerSupport.NONE)
-    public final <@NonNull R> Flowable<R> flatMap(@NonNull Function<? super T, @NonNull ? extends Publisher<? extends R>> mapper,
-            boolean delayErrors, int maxConcurrency, int bufferSize) {
+    public final <@NonNull R> Flowable<R> flatMap(
+            @NonNull Function<? super T, @NonNull ? extends Publisher<? extends R>> mapper,
+            @NonNull FlatMapConfig config) {
         Objects.requireNonNull(mapper, "mapper is null");
-        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
-        ObjectHelper.verifyPositive(bufferSize, "bufferSize");
+        Objects.requireNonNull(config, "config is null");
         if (this instanceof ScalarSupplier) {
             @SuppressWarnings("unchecked")
             T v = ((ScalarSupplier<T>)this).get();
@@ -10427,7 +10309,7 @@ public final Single<T> firstOrError() {
             }
             return FlowableScalarXMap.scalarXMap(v, mapper);
         }
-        return RxJavaPlugins.onAssembly(new FlowableFlatMap<>(this, mapper, delayErrors, maxConcurrency, bufferSize));
+        return RxJavaPlugins.onAssembly(new FlowableFlatMap<>(this, mapper, config.delayErrors(), config.maxConcurrency(), config.bufferSize()));
     }
 
     /**
@@ -10693,7 +10575,7 @@ public final Single<T> firstOrError() {
         Objects.requireNonNull(combiner, "combiner is null");
         ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
         ObjectHelper.verifyPositive(bufferSize, "bufferSize");
-        return flatMap(FlowableInternalHelper.flatMapWithCombiner(mapper, combiner), delayErrors, maxConcurrency, bufferSize);
+        return flatMap(FlowableInternalHelper.flatMapWithCombiner(mapper, combiner), new FlatMapConfig(delayErrors, maxConcurrency, bufferSize));
     }
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java b/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java
new file mode 100644
index 0000000000..3c2e4dfb0a
--- /dev/null
+++ b/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java
@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2016-present, RxJava Contributors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is
+ * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
+ * the License for the specific language governing permissions and limitations under the License.
+ */
+
+package io.reactivex.rxjava4.core.config;
+
+import io.reactivex.rxjava4.core.Flowable;
+import io.reactivex.rxjava4.internal.functions.ObjectHelper;
+
+/**
+ * Generic configuration block with option to delay errors, change prefetch
+ * amounts and buffer sizes.
+ * TODO once value classes are available, make this a record class.
+ * @since 4.0.0
+ */
+public record FlatMapConfig(boolean delayErrors, int maxConcurrency, int bufferSize) {
+
+    /**
+     * Default config: no error delay, {@link Flowable#bufferSize()} sizes.
+     */
+    public FlatMapConfig() {
+        this(false, Flowable.bufferSize(), Flowable.bufferSize());
+    }
+
+    /**
+     * Optionally delay error, {@link Flowable#bufferSize()} sizes 
+     * @param delayErrors should the error be delayed?
+     */
+    public FlatMapConfig(boolean delayErrors) {
+        this(delayErrors, Flowable.bufferSize(), Flowable.bufferSize());
+    }
+
+    /**
+     * Optionally set the buffer size, no delay errors.
+     * @param maxConcurrency the maximum number of concurrent flows
+     */
+    public FlatMapConfig(int maxConcurrency) {
+        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
+        this(false, maxConcurrency, Flowable.bufferSize());
+    }
+
+    /**
+     * Optionally delays errors and sets the buffer size too.
+     * @param delayError 
+     * @param maxConcurrency the maximum number of concurrent flows
+     */
+    public FlatMapConfig(boolean delayErrors, int maxConcurrency) {
+        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
+        this(delayErrors, maxConcurrency, Flowable.bufferSize());
+    }
+
+    /**
+     * Fully customize the configuration. 
+     * @param delayErrors should the errors be delayed
+     * @param bufferSize what would be the buffer size
+     * @param prefetch what would be the prefetch amount
+     */
+    public FlatMapConfig(boolean delayErrors, int maxConcurrency, int bufferSize) {
+        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
+        ObjectHelper.verifyPositive(bufferSize, "bufferSize");
+        this.delayErrors = delayErrors;
+        this.maxConcurrency = maxConcurrency;
+        this.bufferSize = bufferSize;
+    }
+}
diff --git a/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java b/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java
new file mode 100644
index 0000000000..fff26ff638
--- /dev/null
+++ b/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java
@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2016-present, RxJava Contributors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is
+ * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
+ * the License for the specific language governing permissions and limitations under the License.
+ */
+
+package io.reactivex.rxjava4.core.config;
+
+import io.reactivex.rxjava4.core.Flowable;
+import io.reactivex.rxjava4.internal.functions.ObjectHelper;
+
+/**
+ * Generic configuration block with option to delay errors, change prefetch
+ * amounts and buffer sizes.
+ * TODO once value classes are available, make this a record class.
+ * @since 4.0.0
+ */
+public record GenericConfig(boolean delayError, int bufferSize, int prefetch) {
+
+    /**
+     * Default config: no error delay, {@link Flowable#bufferSize()} sizes.
+     */
+    public GenericConfig() {
+        this(false, Flowable.bufferSize(), Flowable.bufferSize());
+    }
+
+    /**
+     * Optionally delay error, {@link Flowable#bufferSize()} sizes 
+     * @param delayError should the error be delayed?
+     */
+    public GenericConfig(boolean delayError) {
+        this(delayError, Flowable.bufferSize(), Flowable.bufferSize());
+    }
+
+    /**
+     * Optionally set the buffer size, no delay errors.
+     * @param bufferSize the prefetch and the buffer size
+     */
+    public GenericConfig(int bufferSize) {
+        ObjectHelper.verifyPositive(bufferSize, "bufferSize");
+        this(false, bufferSize, Flowable.bufferSize());
+    }
+
+    /**
+     * Optionally delays errors and sets the buffer size too.
+     * @param delayError 
+     * @param bufferSize the prefetch and the buffer size
+     */
+    public GenericConfig(boolean delayError, int bufferSize) {
+        ObjectHelper.verifyPositive(bufferSize, "bufferSize");
+        this(delayError, bufferSize, Flowable.bufferSize());
+    }
+
+    /**
+     * Fully customize the configuration. 
+     * @param delayError should the errors be delayed
+     * @param bufferSize what would be the buffer size
+     * @param prefetch what would be the prefetch amount
+     */
+    public GenericConfig(boolean delayError, int bufferSize, int prefetch) {
+        ObjectHelper.verifyPositive(bufferSize, "bufferSize");
+        ObjectHelper.verifyPositive(prefetch, "prefetch");
+        this.delayError = delayError;
+        this.bufferSize = bufferSize;
+        this.prefetch = prefetch;
+    }
+}
diff --git a/src/main/java/io/reactivex/rxjava4/core/docs/FlowableDocBasic.java b/src/main/java/io/reactivex/rxjava4/core/docs/FlowableDocBasic.java
index 357307cddd..33b9e007bc 100644
--- a/src/main/java/io/reactivex/rxjava4/core/docs/FlowableDocBasic.java
+++ b/src/main/java/io/reactivex/rxjava4/core/docs/FlowableDocBasic.java
@@ -19,6 +19,7 @@
 /**
  * Documents a set of operators so that the main Flowable source file is not cluttered.
  * @param <T> the element type of the flow
+ * @since 4.0.0
  */
 public sealed interface FlowableDocBasic<T> permits Flowable {
     /**
diff --git a/src/main/module/module-info.java b/src/main/module/module-info.java
index f59cbe5db8..fb925e8dc4 100644
--- a/src/main/module/module-info.java
+++ b/src/main/module/module-info.java
@@ -14,6 +14,8 @@
 module io.reactivex.rxjava4 {
     exports io.reactivex.rxjava4.annotations;
     exports io.reactivex.rxjava4.core;
+    exports io.reactivex.rxjava4.core.docs;
+    exports io.reactivex.rxjava4.core.config;
     exports io.reactivex.rxjava4.disposables;
     exports io.reactivex.rxjava4.exceptions;
     exports io.reactivex.rxjava4.flowables;
diff --git a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapTest.java b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapTest.java
index 9d22fefa75..cb4890d0bb 100644
--- a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapTest.java
+++ b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableFlatMapTest.java
@@ -27,6 +27,7 @@
 
 import io.reactivex.rxjava4.annotations.NonNull;
 import io.reactivex.rxjava4.core.*;
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.exceptions.*;
 import io.reactivex.rxjava4.functions.*;
 import io.reactivex.rxjava4.internal.functions.Functions;
@@ -344,7 +345,7 @@ public Flowable<Integer> apply(Integer t1) {
                 return composer(Flowable.range(t1 * 10, 2), subscriptionCount, m)
                         .subscribeOn(Schedulers.computation());
             }
-        }, m);
+        }, new FlatMapConfig(m));
 
         TestSubscriber<Integer> ts = new TestSubscriber<>();
 
@@ -585,7 +586,7 @@ public void justEmptyMixtureMaxConcurrent() {
             public Flowable<Integer> apply(Integer v) {
                 return (v & 1) == 0 ? Flowable.<Integer>empty() : Flowable.just(v);
             }
-        }, 16)
+        }, new FlatMapConfig(16))
         .subscribe(ts);
 
         ts.assertValueCount(2 * Flowable.bufferSize());
@@ -610,7 +611,7 @@ public void rangeEmptyMixtureMaxConcurrent() {
             public Flowable<Integer> apply(Integer v) {
                 return (v & 1) == 0 ? Flowable.<Integer>empty() : Flowable.range(v, 2);
             }
-        }, 16)
+        }, new FlatMapConfig(16))
         .subscribe(ts);
 
         ts.assertValueCount(4 * Flowable.bufferSize());
@@ -854,7 +855,7 @@ public Integer apply(Integer w) throws Exception {
                     }
                 });
             }
-        }, true)
+        }, new FlatMapConfig(true))
         .to(TestHelper.<Integer>testConsumer())
         .assertFailure(CompositeException.class);
 
@@ -1063,7 +1064,7 @@ public void maxConcurrencySustained() {
             public Flowable<Integer> apply(PublishProcessor<Integer> v) throws Exception {
                 return v;
             }
-        }, 2)
+        }, new FlatMapConfig(2))
         .doOnNext(new Consumer<Integer>() {
             @Override
             public void accept(Integer v) throws Exception {
@@ -1115,7 +1116,7 @@ public Flowable<Integer> apply(Flowable<Integer> upstream) {
                     public Publisher<Integer> apply(Integer v) throws Throwable {
                         return Flowable.just(v).hide();
                     }
-                }, true);
+                }, new FlatMapConfig(true));
             }
         });
     }
@@ -1250,7 +1251,7 @@ public void fusedInnerCrash() {
                 })
                 .compose(TestHelper.flowableStripBoundary())
         )
-        .flatMap(v -> v, true)
+        .flatMap(v -> v, new FlatMapConfig(true))
         .doOnNext(v -> {
             if (v == 1) {
                 pp.onNext(2);
@@ -1280,7 +1281,7 @@ public void fusedInnerCrash2() {
                 .compose(TestHelper.flowableStripBoundary())
                 , pp
         )
-        .flatMap(v -> v, true)
+        .flatMap(v -> v, new FlatMapConfig(true))
         .doOnNext(v -> {
             if (v == 1) {
                 pp.onNext(2);
@@ -1304,7 +1305,7 @@ public void doubleOnSubscribe() {
     public void allConcurrency() {
         Flowable.just(1)
         .hide()
-        .flatMap(_ -> Flowable.just(2).hide(), Integer.MAX_VALUE)
+        .flatMap(_ -> Flowable.just(2).hide(), new FlatMapConfig(Integer.MAX_VALUE))
         .test()
         .assertResult(2);
     }
@@ -1313,7 +1314,7 @@ public void allConcurrency() {
     public void allConcurrencyScalarInner() {
         Flowable.just(1)
         .hide()
-        .flatMap(_ -> Flowable.just(2), Integer.MAX_VALUE)
+        .flatMap(_ -> Flowable.just(2), new FlatMapConfig(Integer.MAX_VALUE))
         .test()
         .assertResult(2);
     }
@@ -1322,7 +1323,7 @@ public void allConcurrencyScalarInner() {
     public void allConcurrencyScalarInnerEmpty() {
         Flowable.just(1)
         .hide()
-        .flatMap(_ -> Flowable.empty(), Integer.MAX_VALUE)
+        .flatMap(_ -> Flowable.empty(), new FlatMapConfig(Integer.MAX_VALUE))
         .test()
         .assertResult();
     }
@@ -1361,7 +1362,7 @@ public void someConcurrencyScalarInnerCancel() {
     public void allConcurrencyBackpressured() {
         Flowable.just(1)
         .hide()
-        .flatMap(_ -> Flowable.just(2), Integer.MAX_VALUE)
+        .flatMap(_ -> Flowable.just(2), new FlatMapConfig(Integer.MAX_VALUE))
         .test(0L)
         .assertEmpty()
         .requestMore(1)
@@ -1372,7 +1373,7 @@ public void allConcurrencyBackpressured() {
     public void someConcurrencyInnerScalarCancel() {
         Flowable.just(1)
         .hide()
-        .flatMap(_ -> Flowable.just(2), 2)
+        .flatMap(_ -> Flowable.just(2), new FlatMapConfig(2))
         .takeUntil(_ -> true)
         .test()
         .assertResult(2);
@@ -1389,7 +1390,7 @@ protected void subscribeActual(@NonNull Subscriber<@NonNull ? super @NonNull Int
                 subscriber.onNext(3);
             }
         }
-        .flatMap(v -> Flowable.just(v), 1)
+        .flatMap(v -> Flowable.just(v), new FlatMapConfig(1))
         .test(0L)
         .assertFailure(QueueOverflowException.class);
     }
@@ -1405,7 +1406,7 @@ protected void subscribeActual(@NonNull Subscriber<@NonNull ? super @NonNull Int
                 subscriber.onNext(1);
             }
         }
-        .flatMap(v -> Flowable.just(v), 1)
+        .flatMap(v -> Flowable.just(v), new FlatMapConfig(1))
         .doOnNext(v -> {
             if (v == 1) {
                 ref.get().onNext(2);
@@ -1428,7 +1429,7 @@ protected void subscribeActual(@NonNull Subscriber<@NonNull ? super @NonNull Int
                 subscriber.onNext(2);
                 subscriber.onNext(3);
             }
-        }, false, 1, 1)
+        }, new FlatMapConfig(false, 1, 1))
         .test(0L)
         .assertFailure(QueueOverflowException.class);
     }
@@ -1437,7 +1438,7 @@ protected void subscribeActual(@NonNull Subscriber<@NonNull ? super @NonNull Int
     public void takeFromScalarQueue() {
         Flowable.just(1)
         .hide()
-        .flatMap(_ -> Flowable.just(2), 2)
+        .flatMap(_ -> Flowable.just(2), new FlatMapConfig(2))
         .takeUntil(_ -> true)
         .test(0L)
         .requestMore(2)
@@ -1449,7 +1450,7 @@ public void scalarInnerQueueEmpty() {
         Flowable.just(1)
         .concatWith(Flowable.never())
         .hide()
-        .flatMap(_ -> Flowable.just(2), 2)
+        .flatMap(_ -> Flowable.just(2), new FlatMapConfig(2))
         .test(0L)
         .requestMore(2)
         .assertValuesOnly(2);
@@ -1495,8 +1496,7 @@ public void mixedScalarAsync() {
                                 .just(-integer)
                                 .observeOn(Schedulers.computation());
                     },
-                    false,
-                    1
+                    new FlatMapConfig(false, 1)
             )
             .ignoreElements()
             .blockingAwait();
diff --git a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableGroupByTest.java b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableGroupByTest.java
index 6640c77483..9cb751ee8d 100644
--- a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableGroupByTest.java
+++ b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableGroupByTest.java
@@ -31,6 +31,7 @@
 import com.google.common.cache.*;
 
 import io.reactivex.rxjava4.core.*;
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.exceptions.*;
 import io.reactivex.rxjava4.flowables.GroupedFlowable;
 import io.reactivex.rxjava4.functions.*;
@@ -1356,7 +1357,7 @@ public String apply(Integer l) {
                                 }
                             });
                 }
-            }, 4000) // a lot of groups are created due to take(2)
+            }, new FlatMapConfig(4000)) // a lot of groups are created due to take(2)
             .subscribe(ts);
 
         ts.awaitDone(5, TimeUnit.SECONDS);
@@ -2591,7 +2592,7 @@ static void issue6974Run(int groups, int iterations, int sizeCap, int flatMapCon
                 .range(1, groups)
                 .repeat(iterations / groups)
                 .groupBy(i -> i, i -> i, false, 128, sizeCap(sizeCap, notifyOnExplicitRevoke))
-                .flatMap(gf -> gf.compose(operation), flatMapConcurrency)
+                .flatMap(gf -> gf.compose(operation), new FlatMapConfig(flatMapConcurrency))
                 .test();
         test.awaitDone(5, TimeUnit.SECONDS);
         test.assertValueCount(iterations);
@@ -2624,7 +2625,7 @@ static void issue6974RunPart2(int groupByBufferSize, int flatMapMaxConcurrency,
                 sizeCap(groups * 100, notifyOnExplicitEviction))
         .flatMap(gf -> gf
                 .take(10, TimeUnit.MILLISECONDS)
-                , flatMapMaxConcurrency)
+                , new FlatMapConfig(flatMapMaxConcurrency))
         .test();
 
         ts
@@ -2674,7 +2675,7 @@ static void issue6974RunPart2NoEvict(int groupByBufferSize, int flatMapMaxConcur
         .groupBy(i -> i)
         .flatMap(gf -> gf
                 .take(10, TimeUnit.MILLISECONDS)
-                , flatMapMaxConcurrency)
+                , new FlatMapConfig(flatMapMaxConcurrency))
         .subscribeWith(new TestSubscriberEx<>())
         .awaitDone(5, TimeUnit.SECONDS)
         .assertTerminated(); // MBE is possible if the async group closing is slow
@@ -2722,7 +2723,7 @@ public void issue6974Part2Case1ObserveOn() {
                      .observeOn(Schedulers.computation())
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
-            , flatMapMaxConcurrency)
+            , new FlatMapConfig(flatMapMaxConcurrency))
         .subscribeWith(new TestSubscriberEx<>())
         .awaitDone(5, TimeUnit.SECONDS)
         .assertTerminated(); // MBE is possible if the async group closing is slow
@@ -2748,7 +2749,7 @@ public void issue6974Part2Case1ObserveOnHide() {
                      .observeOn(Schedulers.computation())
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
-            , flatMapMaxConcurrency)
+            , new FlatMapConfig(flatMapMaxConcurrency))
         .subscribeWith(new TestSubscriberEx<>())
         .awaitDone(5, TimeUnit.SECONDS)
         .assertTerminated(); // MBE is possible if the async group closing is slow
@@ -2772,7 +2773,7 @@ public void issue6974Part2Case1ObserveOnNoCap() {
                      .observeOn(Schedulers.computation())
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
-            , flatMapMaxConcurrency)
+            , new FlatMapConfig(flatMapMaxConcurrency))
         .test()
         .awaitDone(5, TimeUnit.SECONDS)
         .assertNoErrors()
@@ -2798,7 +2799,7 @@ public void issue6974Part2Case1ObserveOnNoCapHide() {
                      .observeOn(Schedulers.computation())
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
-            , flatMapMaxConcurrency)
+            , new FlatMapConfig(flatMapMaxConcurrency))
         .test()
         .awaitDone(5, TimeUnit.SECONDS)
         .assertNoErrors()
@@ -2835,7 +2836,7 @@ public void issue6974Part2Case1ObserveOnConditional() {
                      .filter(_ -> true)
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
-            , flatMapMaxConcurrency)
+            , new FlatMapConfig(flatMapMaxConcurrency))
         .subscribeWith(new TestSubscriberEx<>())
         .awaitDone(5, TimeUnit.SECONDS)
         .assertTerminated(); // MBE is possible if the async group closing is slow
@@ -2862,7 +2863,7 @@ public void issue6974Part2Case1ObserveOnConditionalHide() {
                      .filter(_ -> true)
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
-            , flatMapMaxConcurrency)
+            , new FlatMapConfig(flatMapMaxConcurrency))
         .subscribeWith(new TestSubscriberEx<>())
         .awaitDone(5, TimeUnit.SECONDS)
         .assertTerminated(); // MBE is possible if the async group closing is slow
@@ -2908,7 +2909,7 @@ public void issue6982Case1() {
         .range(1, 500_000)
         .map(i -> i % groups)
         .groupBy(i -> i, i -> i, false, groupByBufferSize, ttlCapGuava(Duration.ofMillis(10)))
-        .flatMap(gf -> gf.observeOn(Schedulers.computation()), flatMapMaxConcurrency)
+        .flatMap(gf -> gf.observeOn(Schedulers.computation()), new FlatMapConfig(flatMapMaxConcurrency))
         .test()
         .awaitDone(5, TimeUnit.SECONDS)
         .assertNoErrors()
@@ -2938,7 +2939,7 @@ public void issue6982Case2() {
         .range(1, 500_000)
         .map(i -> i % groups)
         .groupBy(i -> i, i -> i, false, groupByBufferSize, ttlCapGuava(Duration.ofMillis(10)))
-        .flatMap(gf -> gf.observeOn(Schedulers.computation()), flatMapMaxConcurrency)
+        .flatMap(gf -> gf.observeOn(Schedulers.computation()), new FlatMapConfig(flatMapMaxConcurrency))
         .test()
         .awaitDone(5, TimeUnit.SECONDS)
         .assertNoErrors()
diff --git a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeTest.java b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeTest.java
index 8c25a755af..f977662af6 100644
--- a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeTest.java
+++ b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableMergeTest.java
@@ -27,6 +27,7 @@
 
 import io.reactivex.rxjava4.core.*;
 import io.reactivex.rxjava4.core.Scheduler.Worker;
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.exceptions.TestException;
 import io.reactivex.rxjava4.functions.*;
 import io.reactivex.rxjava4.internal.functions.Functions;
@@ -1441,7 +1442,7 @@ public void flatMapJustRange() {
     public void flatMapMaxConcurrentJustJust() {
         TestSubscriber<Integer> ts = TestSubscriber.create();
 
-        Flowable.just(Flowable.just(1)).flatMap((Function)Functions.identity(), 5).subscribe(ts);
+        Flowable.just(Flowable.just(1)).flatMap((Function)Functions.identity(), new FlatMapConfig(5)).subscribe(ts);
 
         ts.assertValue(1);
         ts.assertNoErrors();
@@ -1453,7 +1454,7 @@ public void flatMapMaxConcurrentJustJust() {
     public void flatMapMaxConcurrentJustRange() {
         TestSubscriber<Integer> ts = TestSubscriber.create();
 
-        Flowable.just(Flowable.range(1, 5)).flatMap((Function)Functions.identity(), 5).subscribe(ts);
+        Flowable.just(Flowable.range(1, 5)).flatMap((Function)Functions.identity(), new FlatMapConfig(5)).subscribe(ts);
 
         ts.assertValues(1, 2, 3, 4, 5);
         ts.assertNoErrors();
diff --git a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableRetryTest.java b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableRetryTest.java
index 215a066804..b1bbb6e1c3 100644
--- a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableRetryTest.java
+++ b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableRetryTest.java
@@ -26,6 +26,7 @@
 import static java.util.concurrent.Flow.*;
 
 import io.reactivex.rxjava4.core.*;
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.disposables.Disposable;
 import io.reactivex.rxjava4.exceptions.TestException;
 import io.reactivex.rxjava4.flowables.GroupedFlowable;
@@ -864,7 +865,7 @@ public String apply(String t1) {
             public Flowable<String> apply(GroupedFlowable<String, String> t1) {
                 return t1.take(1);
             }
-        }, NUM_MSG) // Must request as many groups as groupBy produces to avoid MBE
+        }, new FlatMapConfig(NUM_MSG)) // Must request as many groups as groupBy produces to avoid MBE
         .subscribe(new TestSubscriber<>(subscriber));
 
         InOrder inOrder = inOrder(subscriber);
diff --git a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWindowWithFlowableTest.java b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWindowWithFlowableTest.java
index d240887306..595eedf98e 100644
--- a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWindowWithFlowableTest.java
+++ b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWindowWithFlowableTest.java
@@ -25,6 +25,7 @@
 import static java.util.concurrent.Flow.*;
 
 import io.reactivex.rxjava4.core.*;
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.exceptions.*;
 import io.reactivex.rxjava4.functions.*;
 import io.reactivex.rxjava4.internal.functions.Functions;
@@ -258,7 +259,7 @@ public void boundaryDispose() {
     public void boundaryOnError() {
         TestSubscriberEx<Object> ts = Flowable.error(new TestException())
         .window(Flowable.never())
-        .flatMap(Functions.<Flowable<Object>>identity(), true)
+        .flatMap(Functions.<Flowable<Object>>identity(), new FlatMapConfig(true))
         .to(TestHelper.<Object>testConsumer())
         .assertFailure(CompositeException.class);
 
diff --git a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWindowWithTimeTest.java b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWindowWithTimeTest.java
index 9c92b85243..a08254de13 100644
--- a/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWindowWithTimeTest.java
+++ b/src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWindowWithTimeTest.java
@@ -23,6 +23,7 @@
 import static java.util.concurrent.Flow.*;
 
 import io.reactivex.rxjava4.core.*;
+import io.reactivex.rxjava4.core.config.FlatMapConfig;
 import io.reactivex.rxjava4.exceptions.*;
 import io.reactivex.rxjava4.functions.*;
 import io.reactivex.rxjava4.internal.functions.Functions;
@@ -249,7 +250,7 @@ public void timespanDefaultSchedulerSize() {
     public void timespanDefaultSchedulerSizeRestart() {
         Flowable.range(1, 10)
         .window(1, TimeUnit.MINUTES, 20, true)
-        .flatMap(Functions.<Flowable<Integer>>identity(), true)
+        .flatMap(Functions.<Flowable<Integer>>identity(), new FlatMapConfig(true))
         .test()
         .awaitDone(5, TimeUnit.SECONDS)
         .assertResult(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
diff --git a/src/test/java/io/reactivex/rxjava4/validators/ParamValidationCheckerTest.java b/src/test/java/io/reactivex/rxjava4/validators/ParamValidationCheckerTest.java
index bc4f3fea23..8c570f7552 100644
--- a/src/test/java/io/reactivex/rxjava4/validators/ParamValidationCheckerTest.java
+++ b/src/test/java/io/reactivex/rxjava4/validators/ParamValidationCheckerTest.java
@@ -17,14 +17,15 @@
 import java.time.Duration;
 import java.util.*;
 import java.util.concurrent.*;
+import java.util.concurrent.Flow.*;
 import java.util.stream.*;
 
 import org.junit.Test;
-import static java.util.concurrent.Flow.*;
 
 import io.reactivex.rxjava4.core.*;
 import io.reactivex.rxjava4.core.Observable;
 import io.reactivex.rxjava4.core.Observer;
+import io.reactivex.rxjava4.core.config.*;
 import io.reactivex.rxjava4.disposables.*;
 import io.reactivex.rxjava4.exceptions.TestException;
 import io.reactivex.rxjava4.functions.*;
@@ -612,6 +613,9 @@ public void checkParallelFlowable() {
         VirtualGenerator<Object> vg = _ -> { };
         defaultValues.put(VirtualGenerator.class, vg);
 
+        defaultValues.put(FlatMapConfig.class, new FlatMapConfig());
+        defaultValues.put(GenericConfig.class, new GenericConfig());
+
         @SuppressWarnings("rawtypes")
         class MixedConverters implements FlowableConverter, ObservableConverter, SingleConverter,
         MaybeConverter, CompletableConverter, ParallelFlowableConverter {

From 6df9a6ef6d550a8224b2904bf591f2c85167d8b2 Mon Sep 17 00:00:00 2001
From: akarnokd <akarnokd@gmail.com>
Date: Thu, 19 Mar 2026 12:50:54 +0100
Subject: [PATCH 2/3] Missed these

---
 config/checkstyle/suppressions.xml                     |  4 ----
 .../reactivex/rxjava4/core/config/FlatMapConfig.java   | 10 ++++++----
 .../reactivex/rxjava4/core/config/GenericConfig.java   |  6 ++----
 3 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/config/checkstyle/suppressions.xml b/config/checkstyle/suppressions.xml
index 0d28559996..f4675fd64c 100644
--- a/config/checkstyle/suppressions.xml
+++ b/config/checkstyle/suppressions.xml
@@ -12,8 +12,4 @@
   <suppress checks="MissingJavadocMethod|JavadocMethod" files=".*Test\.java$"/>
   <suppress checks="MissingJavadocMethod|JavadocMethod" files=".*Perf\.java$"/>
 
-  <!-- FIXME Checkstyle Bug: https://github.com/checkstyle/checkstyle/issues/19342 -->
-  <suppress checks=".*" files=".*[\\/]FlatMapConfig\.java$"/>
-  <suppress checks=".*" files=".*[\\/]GenericConfig\.java$"/>
-  
 </suppressions>
diff --git a/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java b/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java
index 3c2e4dfb0a..3b6c3d7718 100644
--- a/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java
+++ b/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java
@@ -44,8 +44,9 @@ public FlatMapConfig(boolean delayErrors) {
      * @param maxConcurrency the maximum number of concurrent flows
      */
     public FlatMapConfig(int maxConcurrency) {
-        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
-        this(false, maxConcurrency, Flowable.bufferSize());
+        this(false, 
+                ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency"), 
+                Flowable.bufferSize());
     }
 
     /**
@@ -54,8 +55,9 @@ public FlatMapConfig(int maxConcurrency) {
      * @param maxConcurrency the maximum number of concurrent flows
      */
     public FlatMapConfig(boolean delayErrors, int maxConcurrency) {
-        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
-        this(delayErrors, maxConcurrency, Flowable.bufferSize());
+        this(delayErrors, 
+                ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency"), 
+                Flowable.bufferSize());
     }
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java b/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java
index fff26ff638..078148abdd 100644
--- a/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java
+++ b/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java
@@ -44,8 +44,7 @@ public GenericConfig(boolean delayError) {
      * @param bufferSize the prefetch and the buffer size
      */
     public GenericConfig(int bufferSize) {
-        ObjectHelper.verifyPositive(bufferSize, "bufferSize");
-        this(false, bufferSize, Flowable.bufferSize());
+        this(false, ObjectHelper.verifyPositive(bufferSize, "bufferSize"), Flowable.bufferSize());
     }
 
     /**
@@ -54,8 +53,7 @@ public GenericConfig(int bufferSize) {
      * @param bufferSize the prefetch and the buffer size
      */
     public GenericConfig(boolean delayError, int bufferSize) {
-        ObjectHelper.verifyPositive(bufferSize, "bufferSize");
-        this(delayError, bufferSize, Flowable.bufferSize());
+        this(delayError, ObjectHelper.verifyPositive(bufferSize, "bufferSize"), Flowable.bufferSize());
     }
 
     /**

From 0b15d3c164fb967a67eb5defd2b91bb0f1531c64 Mon Sep 17 00:00:00 2001
From: akarnokd <akarnokd@gmail.com>
Date: Thu, 19 Mar 2026 12:57:53 +0100
Subject: [PATCH 3/3] Apply checkstyle fixes

---
 .../rxjava4/core/config/FlatMapConfig.java         | 14 +++++++-------
 .../rxjava4/core/config/GenericConfig.java         |  6 +++---
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java b/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java
index 3b6c3d7718..2506cdf3a3 100644
--- a/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java
+++ b/src/main/java/io/reactivex/rxjava4/core/config/FlatMapConfig.java
@@ -32,7 +32,7 @@ public FlatMapConfig() {
     }
 
     /**
-     * Optionally delay error, {@link Flowable#bufferSize()} sizes 
+     * Optionally delay error, {@link Flowable#bufferSize()} sizes
      * @param delayErrors should the error be delayed?
      */
     public FlatMapConfig(boolean delayErrors) {
@@ -44,24 +44,24 @@ public FlatMapConfig(boolean delayErrors) {
      * @param maxConcurrency the maximum number of concurrent flows
      */
     public FlatMapConfig(int maxConcurrency) {
-        this(false, 
-                ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency"), 
+        this(false,
+                ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency"),
                 Flowable.bufferSize());
     }
 
     /**
      * Optionally delays errors and sets the buffer size too.
-     * @param delayError 
+     * @param delayError should the errors be delayed?
      * @param maxConcurrency the maximum number of concurrent flows
      */
     public FlatMapConfig(boolean delayErrors, int maxConcurrency) {
-        this(delayErrors, 
-                ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency"), 
+        this(delayErrors,
+                ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency"),
                 Flowable.bufferSize());
     }
 
     /**
-     * Fully customize the configuration. 
+     * Fully customize the configuration.
      * @param delayErrors should the errors be delayed
      * @param bufferSize what would be the buffer size
      * @param prefetch what would be the prefetch amount
diff --git a/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java b/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java
index 078148abdd..84e6929034 100644
--- a/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java
+++ b/src/main/java/io/reactivex/rxjava4/core/config/GenericConfig.java
@@ -32,7 +32,7 @@ public GenericConfig() {
     }
 
     /**
-     * Optionally delay error, {@link Flowable#bufferSize()} sizes 
+     * Optionally delay error, {@link Flowable#bufferSize()} sizes.
      * @param delayError should the error be delayed?
      */
     public GenericConfig(boolean delayError) {
@@ -49,7 +49,7 @@ public GenericConfig(int bufferSize) {
 
     /**
      * Optionally delays errors and sets the buffer size too.
-     * @param delayError 
+     * @param delayError should the errors be delayed?
      * @param bufferSize the prefetch and the buffer size
      */
     public GenericConfig(boolean delayError, int bufferSize) {
@@ -57,7 +57,7 @@ public GenericConfig(boolean delayError, int bufferSize) {
     }
 
     /**
-     * Fully customize the configuration. 
+     * Fully customize the configuration.
      * @param delayError should the errors be delayed
      * @param bufferSize what would be the buffer size
      * @param prefetch what would be the prefetch amount