extension function in java

core-kotlin/src/main/kotlin/com/baeldung/kotlin/ListUtil.kt

@@ -0,0 +1,9 @@
+@file:JvmName("Lists")
+package com.baeldung.kotlin
+
+fun <T> MutableList<T>.swap(firstIndex: Int, secondIndex: Int): MutableList<T> {
+    val tmp = this[firstIndex]
+    this[firstIndex] = this[secondIndex]
+    this[secondIndex] = tmp
+    return this
+}

core-kotlin/src/test/java/com/baeldung/kotlin/ListUtilTest.java

@@ -0,0 +1,22 @@
+package com.baeldung.kotlin;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.List;
+
+import static com.baeldung.kotlin.Lists.*;
+
+public class ListUtilTest {
+
+    @Test
+    public void shouldSwapTwoElementsInList() {
+        List<Integer> list = Arrays.asList(0, 1, 2);
+
+        List<Integer> swappedElements = swap(list, 1, 2);
+
+        Assert.assertEquals(swappedElements, Arrays.asList(0, 2, 1));
+    }
+
+}