diff --git a/s-day07/src/com/inmind/stream07/CountDemo16.java b/s-day07/src/com/inmind/stream07/CountDemo16.java
new file mode 100644
index 0000000..08e8a75
--- /dev/null
+++ b/s-day07/src/com/inmind/stream07/CountDemo16.java
@@ -0,0 +1,18 @@
+package com.inmind.stream07;
+
+import java.util.stream.Stream;
+
+/*
+Stream流中的常用方法_count
+long count() 返回此流中的元素数
+流是一个流水生产线，是不能倒退的。count()是一个终结方法，就是这个流结束，那个该流就无法再操作
+ */
+public class CountDemo16 {
+    public static void main(String[] args) {
+        Stream<Integer> stream = Stream.of(1, 2, 3, 4, 5);
+        long count = stream.count();
+        System.out.println(count);
+        //java.lang.IllegalStateException: stream has already been operated upon or closed
+       // stream.forEach(i -> System.out.println(i));//错误，因为流已经结束，不能再操作了
+    }
+}