diff --git a/docs/notes/20. 接口优于抽象类.md b/docs/notes/20. 接口优于抽象类.md
index daebfab..afb63f2 100644
--- a/docs/notes/20. 接口优于抽象类.md	
+++ b/docs/notes/20. 接口优于抽象类.md	
@@ -53,7 +53,7 @@ static List<Integer> intArrayAsList(int[] a) {
 
         @Override 
         public Integer set(int i, Integer val) {
-            int oldVal = a[I];
+            int oldVal = a[i];
             a[i] = val;     // Auto-unboxing
             return oldVal;  // Autoboxing
         }