[FLINK-37383]Add test coverage for ThrottledIterator edge cases

Adds test coverage for invalid elements per second, consistent window
size, and non-serializable source scenarios in ThrottledIterator tests.

flink-examples/flink-examples-streaming/src/test/java/org/apache/flink/streaming/test/examples/utils/ThrottledIteratorITCase.java

@@ -0,0 +1,150 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.flink.streaming.test.examples.utils;
+
+import org.apache.flink.streaming.examples.utils.ThrottledIterator;
+import org.apache.flink.util.TestLogger;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Supplier;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+/** Tests for {@link ThrottledIterator}. */
+public class ThrottledIteratorITCase extends TestLogger {
+
+    @Test
+    public void testThrottledIteratorRespectsElementsPerSecondRate() throws Exception {
+        AtomicLong currentTime = new AtomicLong(1000L);
+        AtomicLong sleepTime = new AtomicLong();
+
+        Supplier<Long> timeSupplier = currentTime::get;
+        ThrottledIterator.SleepFunction sleepFunction = sleepTime::set;
+
+        // Create test data
+        Iterator<Integer> source = Arrays.asList(1, 2, 3, 4, 5).iterator();
+        ThrottledIterator<Integer> throttledIterator =
+                new ThrottledIterator<>(source, 2, timeSupplier, sleepFunction);
+
+        // Simulate time progression
+        currentTime.set(1300L);
+        assertEquals(Integer.valueOf(1), throttledIterator.next());
+
+        currentTime.set(1700L);
+        assertEquals(Integer.valueOf(2), throttledIterator.next());
+
+        currentTime.set(2000L);
+        assertEquals(Integer.valueOf(3), throttledIterator.next());
+
+        // Verify sleep was called
+        assertEquals(50L, sleepTime.get());
+    }
+
+    @Test
+    public void testThrottledIteratorHasConsistentWindowSizes() throws Exception {
+        AtomicLong currentTime = new AtomicLong(1000L);
+        AtomicLong sleepTime = new AtomicLong();
+
+        Supplier<Long> timeSupplier = currentTime::get;
+        ThrottledIterator.SleepFunction sleepFunction = sleepTime::set;
+
+        // Create iterator that returns incrementing sequence
+        Iterator<Integer> source =
+                new Iterator<>() {
+                    private int count = 0;
+
+                    @Override
+                    public boolean hasNext() {
+                        return true;
+                    }
+
+                    @Override
+                    public Integer next() {
+                        return ++count;
+                    }
+                };
+        ThrottledIterator<Integer> throttledIterator =
+                new ThrottledIterator<>(source, 1, timeSupplier, sleepFunction);
+
+        // Simulate time progression and consume elements
+        currentTime.set(2020L);
+        throttledIterator.next();
+
+        currentTime.set(3040L);
+        throttledIterator.next();
+
+        currentTime.set(4030L);
+        throttledIterator.next();
+        assertEquals(50L, sleepTime.get()); // First window sleep
+
+        currentTime.set(5100L);
+        throttledIterator.next();
+
+        currentTime.set(6120L);
+        throttledIterator.next();
+
+        currentTime.set(7140L);
+        throttledIterator.next();
+
+        currentTime.set(8150L);
+        throttledIterator.next();
+        assertEquals(50L, sleepTime.get()); // Second window sleep - same size
+    }
+
+    @Test
+    public void testThrottledIteratorWithInvalidElementsPerSecond() {
+        Iterator<Integer> source = Collections.emptyIterator();
+
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> new ThrottledIterator<>(source, 0),
+                "'elements per second' must be positive and not zero");
+
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> new ThrottledIterator<>(source, -1),
+                "'elements per second' must be positive and not zero");
+    }
+
+    @Test
+    public void testThrottledIteratorWithNonSerializableSource() {
+        Iterator<Integer> nonSerializableSource =
+                new Iterator<Integer>() {
+                    @Override
+                    public boolean hasNext() {
+                        return true;
+                    }
+
+                    @Override
+                    public Integer next() {
+                        return 1;
+                    }
+                };
+
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> new ThrottledIterator<>(nonSerializableSource, 1),
+                "source must be java.io.Serializable");
+    }
+}