Enhance docs, add tests in Intersection (#5976)

src/test/java/com/thealgorithms/datastructures/hashmap/hashing/IntersectionTest.java

@@ -0,0 +1,76 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class IntersectionTest {
+
+    @Test
+    void testBasicIntersection() {
+        int[] arr1 = {1, 2, 2, 1};
+        int[] arr2 = {2, 2};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertEquals(List.of(2, 2), result, "Intersection should return [2, 2]");
+    }
+
+    @Test
+    void testNoIntersection() {
+        int[] arr1 = {1, 2, 3};
+        int[] arr2 = {4, 5, 6};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertTrue(result.isEmpty(), "Intersection should be empty for disjoint sets");
+    }
+
+    @Test
+    void testEmptyArray() {
+        int[] arr1 = {};
+        int[] arr2 = {1, 2, 3};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertTrue(result.isEmpty(), "Intersection should be empty when first array is empty");
+
+        result = Intersection.intersection(arr2, arr1);
+        assertTrue(result.isEmpty(), "Intersection should be empty when second array is empty");
+    }
+
+    @Test
+    void testNullArray() {
+        int[] arr1 = null;
+        int[] arr2 = {1, 2, 3};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertTrue(result.isEmpty(), "Intersection should be empty when first array is null");
+
+        result = Intersection.intersection(arr2, arr1);
+        assertTrue(result.isEmpty(), "Intersection should be empty when second array is null");
+    }
+
+    @Test
+    void testMultipleOccurrences() {
+        int[] arr1 = {5, 5, 5, 6};
+        int[] arr2 = {5, 5, 6, 6, 6};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertEquals(List.of(5, 5, 6), result, "Intersection should return [5, 5, 6]");
+    }
+
+    @Test
+    void testSameElements() {
+        int[] arr1 = {1, 1, 1};
+        int[] arr2 = {1, 1, 1};
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertEquals(List.of(1, 1, 1), result, "Intersection should return [1, 1, 1] for same elements");
+    }
+
+    @Test
+    void testLargeArrays() {
+        int[] arr1 = new int[1000];
+        int[] arr2 = new int[1000];
+        for (int i = 0; i < 1000; i++) {
+            arr1[i] = i;
+            arr2[i] = i;
+        }
+        List<Integer> result = Intersection.intersection(arr1, arr2);
+        assertEquals(1000, result.size(), "Intersection should return all elements for identical large arrays");
+    }
+}