basler1337@gmail.com feat add element to array list at specific index (#13025)

* feat add element to array list at specific index

* add unit test for add element at index

core-java-modules/core-java-collections-list-4/src/test/java/com/baeldung/list/addelementatindex/AddElementAtIndexUnitTest.java

@@ -0,0 +1,40 @@
+package com.baeldung.list.addelementatindex;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+
+class AddElementAtIndexUnitTest {
+
+    @Test
+    void givenArrayList_whenAddElementAtIndex_thenGetExpectedElementAtIndex() {
+        List<Integer> integers = new ArrayList<>();
+        // add some elements
+        integers.add(5);
+        integers.add(6);
+        integers.add(7);
+        integers.add(8);
+        assertThat(integers.get(0)).isEqualTo(5);
+        assertThat(integers.get(1)).isEqualTo(6);
+        assertThat(integers.get(2)).isEqualTo(7);
+        assertThat(integers.get(3)).isEqualTo(8);
+        // add an element at a specific index
+        integers.add(1, 9);
+        assertThat(integers.get(0)).isEqualTo(5);
+        assertThat(integers.get(1)).isEqualTo(9);
+        assertThat(integers.get(2)).isEqualTo(6);
+        assertThat(integers.get(3)).isEqualTo(7);
+        assertThat(integers.get(4)).isEqualTo(8);
+        // sort the array list
+        integers.sort(Integer::compareTo);
+        assertThat(integers.get(0)).isEqualTo(5);
+        assertThat(integers.get(1)).isEqualTo(6);
+        assertThat(integers.get(2)).isEqualTo(7);
+        assertThat(integers.get(3)).isEqualTo(8);
+        assertThat(integers.get(4)).isEqualTo(9);
+    }
+}