diff --git a/docs/kotlin.md b/docs/kotlin.md
index 7532156..a70d074 100644
--- a/docs/kotlin.md
+++ b/docs/kotlin.md
@@ -678,6 +678,78 @@ fun main(args: Array<String>) {
 这里之所以可以把lambda写在外部，是因为operation是最后一个参数。
 <!--rehype:className=wrap-text-->
 
+### 扩展函数
+
+```kotlin
+// Kotlin File
+fun String.lettersCount(): Int {
+    var count = 0
+    // this 相当于我们下面写的字符串具体的内容
+    // for 可以用 forEach 代替
+    for (char in this) {
+        // 判断是不是字母（包括中文）
+        if (char.isLetter()) {
+            count++
+        }
+    }
+    return count
+}
+
+fun main() {
+    //不修改 String 类的情况下新增方法
+    println("123demo".lettersCount())
+    // Print: 4
+}
+```
+
+### 运算符重载
+
+```kotlin
+class Money(var amount: Double)
+
+// 配合扩展函数，重载运算符 + 即 plus
+operator fun Money.plus(money: Money): Money {
+    // 把金额相加返回一个新的 Money对象
+    return Money(this.amount + money.amount)
+}
+
+fun main() {
+    val appleMoney = Money(10.0)
+    val eggMoney = Money(6.0)
+    // 你没有看错，我们将两个类对象相加了
+    val allMoney = appleMoney + eggMoney
+    println(allMoney.amount)
+    // Print: 16.0
+}
+
+```
+
+这里的 **运算符重载** 依赖于 **扩展函数**
+<!--rehype:className=wrap-text-->
+
+### 中缀表达式
+
+```kotlin
+// infix 定义一个中缀表达式，类似扩展函数那样
+infix fun LocalDate.formatBy(pattern:String):String{
+    val formatter =  DateTimeFormatter.ofPattern(pattern)
+    return this.format(formatter)
+
+}
+
+fun main() {
+    val currentDate = LocalDate.now()
+    println(currentDate formatBy "yyyy-MM-dd")
+    // Print: 2024-02-08
+
+    (1 until  100).forEach { 
+        println(it)
+        // Print 1 至 99
+    }
+}
+
+```
+
 类
 ---
 
@@ -707,20 +779,52 @@ fun main() {
 <!--rehype:wrap-class=col-span-2-->
 
 ```kotlin
-class Student(val name: String, val gpa: Double, val semester: String, val estimatedGraduationYear: Int) 
+class Student(
+    val name: String,
+    val gpa: Double,
+    val semester: String,
+    val estimatedGraduationYear: Int
+)
 
 fun main() {
-  var student = Student("Lucia", 3.95, "Fall", 2022) 
-  println(student.name)     
-  // Prints: Lucia
-  println(student.gpa)      
-  // Prints: 3.95
-  println(student.semester) 
-  // Prints: Fall
-  println(student.estimatedGraduationYear) 
-  // Prints: 2022
+    val student = Student("Lucia", 3.95, "Fall", 2022)
+    println(student.name)
+    // Prints: Lucia
+    println(student.gpa)
+    // Prints: 3.95
+    println(student.semester)
+    // Prints: Fall
+    println(student.estimatedGraduationYear)
+    // Prints: 2022
 }
+
 ```
+
+### 次构造函数
+<!--rehype:wrap-class=col-span-2-->
+
+```kotlin
+class Student(
+    val name: String,
+    val gpa: Double,
+    val semester: String,
+    val estimatedGraduationYear: Int
+) {
+    constructor(name: String, gpa: Double) : this(name, gpa, "Fall", 2024)
+}
+
+fun main() {
+    val student = Student("Lucia", 3.95)
+    println(student.name)
+    // Prints: Lucia
+    println(student.semester)
+    // Prints: Fall
+    println(student.estimatedGraduationYear)
+    // Prints: 2024
+}
+
+```
+
 <!--rehype:className=wrap-text-->
 
 ### 类示例
@@ -785,6 +889,93 @@ fun main() {
 ```
 <!--rehype:className=wrap-text-->
 
+### Data数据类
+
+```kotlin
+// 默认实现 getter/setter 和 toString 这些方法
+data class UserInfo(
+    val name: String,
+    val age: Int
+)
+
+fun main() {
+
+    val userInfo = UserInfo("张三", 20)
+    println(userInfo.name)
+    // 张三
+    println(userInfo.toString())
+    // UserInfo(name=张三, age=20)
+}
+```
+
+### 伴生对象
+
+```kotlin
+// 私有化构造方法
+class User private constructor(val name: String) {
+    // 伴生对象，相当于一个静态类
+    companion object {
+        fun createUser(name: String): User {
+            return User(name)
+        }
+    }
+}
+
+fun main() {
+    // 就像是调用静态方法
+    val user = User.createUser("张三")
+    println(user.name)
+    //Print: 张三
+}
+
+```
+
+### 内部类
+
+```kotlin
+class Outer {
+    val outStr: String = "Outer"
+    // inner 可以让内部类访问外部类
+    inner class Inner {
+        fun printOutStr(){
+            println(outStr)
+        }
+    }
+}
+
+fun main() {
+    val outer = Outer()
+    outer.Inner().printOutStr()
+    // Print: Outer
+}
+```
+
+如果不用inner修饰，会导致Inner类无法使用outStr
+<!--rehype:className=wrap-text-->
+
+### object单例类
+
+```kotlin
+object HttpUtils {
+
+    const val baseUrl = "https://xxxx.com"
+
+    fun getRequest(url: String): String {
+        // 示例代码....
+        return "Result"
+    }
+}
+
+fun main() {
+    println(HttpUtils.baseUrl)
+    // Print: "https://xxxx.com"
+    HttpUtils.getRequest("xxxxx")
+}
+```
+
+object类中定义的函数和属性都可以用类名直接引用
+<!--rehype:className=wrap-text-->
+
 另见
 ---