setTitle method not working in Child Activity

I'm new to Android development. Normally in any activity class, I just simply use setTitle("My title") at any place will do then the window title will change dynamically.

However, when I try using this method in child activity, it won't have any effect. When I try 开发者_运维知识库use Toast, Menu option it also no effect at all.

How can this be done?