diff --git a/src/basic/Variable.kt b/src/basic/Variable.kt
index 9ff175a..091c087 100644
--- a/src/basic/Variable.kt
+++ b/src/basic/Variable.kt
@@ -99,4 +99,13 @@ fun main() {
         it == 6L
     }
     println(index)
+
+    // 可空变量
+    var str: String? = null
+    // 安全调用符
+    println(str?.length)
+    // 默认值
+    println(str?.length ?: "默认值")
+    // 非空断言 为空 运行时抛空指针异常
+    println(str!!.length)
 }
\ No newline at end of file