Create kotlin.md

categories/kotlin.md

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+# Kotlin
+
+## Some tips
+
+### Builder mode in kotlin
+
+```kotlin
+class Car(
+  val model: String?,
+  val color: String?,
+  val type: String?) {
+
+    data class Builder(
+      var model: String? = null,
+      var color: String = "pink",
+      var type: String? = null) {
+
+        fun model(model: String) = apply { this.model = model }
+        fun color(color: String) = apply { this.color = color }
+        fun type(type: String) = apply { this.type = type }
+        fun build() = Car(model, color, type)
+    }
+}
+//use
+val car = Car.Builder()
+  .model("Ford Focus")
+  .color("Black")
+  .type("Type")
+  .build()
+```
+Based on [stackoverflow](https://stackoverflow.com/questions/36140791/how-to-implement-builder-pattern-in-kotlin)