BAEL-2305 Adds array intersection operations

core-java-arrays/src/main/java/com/baeldung/array/operations/ArrayOperations.java

@@ -4,11 +4,13 @@ import java.lang.reflect.Array;
 import java.util.Arrays;
 import java.util.HashSet;
 import java.util.LinkedHashSet;
+import java.util.LinkedList;
 import java.util.Random;
 import java.util.Set;
 import java.util.function.Function;
 import java.util.function.IntPredicate;
 import java.util.function.Predicate;
+import java.util.stream.Stream;
 
 import org.apache.commons.lang3.ArrayUtils;
 
@@ -194,4 +196,16 @@ public class ArrayOperations {
     public static <T> T getRandomFromObjectArray(T[] array) {
         return array[new Random().nextInt(array.length)];
     }
+
+    public static Integer[] intersectionSimple(final Integer[] a, final Integer[] b){
+        return Stream.of(a).filter(Arrays.asList(b)::contains).toArray(Integer[]::new);
+    }
+
+    public static Integer[] intersectionSet(final Integer[] a, final Integer[] b){
+        return Stream.of(a).filter(Arrays.asList(b)::contains).distinct().toArray(Integer[]::new);
+    }
+
+    public static Integer[] intersectionMultiSet(final Integer[] a, final Integer[] b){
+        return Stream.of(a).filter(new LinkedList<>(Arrays.asList(b))::remove).toArray(Integer[]::new);
+    }
 }

core-java-arrays/src/test/java/com/baeldung/array/operations/IntersectionUnitTest.java

@@ -0,0 +1,66 @@
+package com.baeldung.array.operations;
+
+import org.junit.jupiter.api.Test;
+
+import static com.baeldung.array.operations.ArrayOperations.intersectionMultiSet;
+import static com.baeldung.array.operations.ArrayOperations.intersectionSet;
+import static com.baeldung.array.operations.ArrayOperations.intersectionSimple;
+import static org.assertj.core.api.Assertions.assertThat;
+
+class IntersectionUnitTest {
+    private static final Integer[] a = { 1, 3, 2 };
+    private static final Integer[] b = { 4, 3, 2, 4, 2, 3, 4, 4, 3 };
+    private static final Integer[] c = { 1, 3, 2, 3, 3, 2 };
+
+    @Test
+    void whenIntersectionSimpleIsUsed_thenCommonEntriesAreInTheResult() {
+        assertThat(intersectionSimple(a, b)).isEqualTo(new Integer[] { 3, 2 });
+        assertThat(intersectionSimple(b, a)).isEqualTo(new Integer[] { 3, 2, 2, 3, 3 });
+    }
+
+    @Test
+    void whenIntersectionSimpleIsUsedWithAnArrayAndItself_thenTheResultIsTheIdentity() {
+        assertThat(intersectionSimple(b, b)).isEqualTo(b);
+        assertThat(intersectionSimple(a, a)).isEqualTo(a);
+    }
+
+    @Test
+    void whenIntersectionSetIsUsed_thenCommonEntriesAreInTheResult() {
+        assertThat(intersectionSet(b, a)).isEqualTo(new Integer[] { 3, 2 });
+    }
+
+    @Test
+    void whenIntersectionSetIsUsed_thenTheNumberOfEntriesDoesNotChangeWithTheParameterOrder() {
+        assertThat(intersectionSet(a, b)).isEqualTo(new Integer[] { 3, 2 });
+        assertThat(intersectionSet(b, a)).isEqualTo(new Integer[] { 3, 2 });
+    }
+
+    @Test
+    void whenIntersectionSetIsUsedWithAnArrayAndWithItself_andTheInputHasNoDuplicateEntries_ThenTheResultIsTheIdentity() {
+        assertThat(intersectionSet(a, a)).isEqualTo(a);
+    }
+
+    @Test
+    void whenIntersectionSetIsUsedWithAnArrayAndWithItself_andTheInputHasDuplicateEntries_ThenTheResultIsNotTheIdentity() {
+        assertThat(intersectionSet(b, b)).isNotEqualTo(b);
+    }
+
+    @Test
+    void whenMultiSetIsUsed_thenCommonEntriesAreInTheResult() {
+        assertThat(intersectionMultiSet(b, a)).isEqualTo(new Integer[] { 3, 2 });
+    }
+
+    @Test
+    void whenIntersectionMultiSetIsUsed_thenTheNumberOfEntriesDoesNotChangeWithTheParameterOrder() {
+        assertThat(intersectionMultiSet(a, b)).isEqualTo(new Integer[] { 3, 2 });
+        assertThat(intersectionMultiSet(b, a)).isEqualTo(new Integer[] { 3, 2 });
+        assertThat(intersectionMultiSet(b, c)).isEqualTo(new Integer[] { 3, 2, 2, 3, 3 });
+        assertThat(intersectionMultiSet(c, b)).isEqualTo(new Integer[] { 3, 2, 3, 3, 2 });
+    }
+
+    @Test
+    void whenIntersectionMultiSetIsUsedWithAnArrayAndWithItself_ThenTheResultIsTheIdentity() {
+        assertThat(intersectionMultiSet(b, b)).isEqualTo(b);
+        assertThat(intersectionMultiSet(a, a)).isEqualTo(a);
+    }
+}