Add ReservoirSampling algorithm to randomized module (#6204)

src/test/java/com/thealgorithms/randomized/ReservoirSamplingTest.java

@@ -0,0 +1,45 @@
+package com.thealgorithms.randomized;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.Arrays;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class ReservoirSamplingTest {
+
+    @Test
+    public void testSampleSizeEqualsStreamLength() {
+        int[] stream = {1, 2, 3, 4, 5};
+        int sampleSize = 5;
+
+        List<Integer> result = ReservoirSampling.sample(stream, sampleSize);
+
+        assertEquals(sampleSize, result.size());
+        assertTrue(Arrays.stream(stream).allMatch(result::contains));
+    }
+
+    @Test
+    public void testSampleSizeLessThanStreamLength() {
+        int[] stream = {10, 20, 30, 40, 50, 60};
+        int sampleSize = 3;
+
+        List<Integer> result = ReservoirSampling.sample(stream, sampleSize);
+
+        assertEquals(sampleSize, result.size());
+        for (int value : result) {
+            assertTrue(Arrays.stream(stream).anyMatch(x -> x == value));
+        }
+    }
+
+    @Test
+    public void testSampleSizeGreaterThanStreamLengthThrowsException() {
+        int[] stream = {1, 2, 3};
+
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> ReservoirSampling.sample(stream, 5));
+
+        assertEquals("Sample size cannot exceed stream size.", exception.getMessage());
+    }
+}