Update Java程序设计题.md

docs/java/Java程序设计题.md

@@ -73,7 +73,7 @@ public class MyStack {
 
     //TODO：返回栈顶元素并出栈
     private int pop() {
-        if (count == -1)
+        if (count == 0)
             throw new IllegalArgumentException("Stack is empty.");
         count--;
         return storage[count];