Examples of writing Extension Methods

core-kotlin/src/test/kotlin/com/baeldung/kotlin/ExtensionMethods.kt

@@ -0,0 +1,51 @@
+package com.baeldung.kotlin
+
+import org.junit.Assert
+import org.junit.Test
+
+class ExtensionMethods {
+    @Test
+    fun simpleExtensionMethod() {
+        fun String.escapeForXml() : String {
+            return this
+                    .replace("&", "&")
+                    .replace("<", "&lt;")
+                    .replace(">", "&gt;")
+        }
+
+        Assert.assertEquals("Nothing", "Nothing".escapeForXml())
+        Assert.assertEquals("&lt;Tag&gt;", "<Tag>".escapeForXml())
+        Assert.assertEquals("a&amp;b", "a&b".escapeForXml())
+    }
+
+    @Test
+    fun genericExtensionMethod() {
+        fun <T> T.concatAsString(b: T) : String {
+            return this.toString() + b.toString()
+        }
+
+        Assert.assertEquals("12", "1".concatAsString("2"))
+        Assert.assertEquals("12", 1.concatAsString(2))
+        // This doesn't compile
+        // Assert.assertEquals("12", 1.concatAsString(2.0))
+    }
+
+    @Test
+    fun infixExtensionMethod() {
+        infix fun Number.toPowerOf(exponent: Number): Double {
+            return Math.pow(this.toDouble(), exponent.toDouble())
+        }
+
+        Assert.assertEquals(9.0, 3 toPowerOf 2, 0.1)
+        Assert.assertEquals(3.0, 9 toPowerOf 0.5, 0.1)
+    }
+
+    @Test
+    fun operatorExtensionMethod() {
+        operator fun List<Int>.times(by: Int): List<Int> {
+            return this.map { it * by }
+        }
+
+        Assert.assertEquals(listOf(2, 4, 6), listOf(1, 2, 3) * 2)
+    }
+}