继承JFrame出错哦，下面是显示的错误的信息：
Access restriction: The type JFrame is not accessible due to restriction on required library D:\Program Files\Java\jdk1.8.0_40\jre\lib\rt.jar
解决办法：
右键工程目录选择properties 选择java build path ->libraries ->JRE System Library->Edit->Alternate JRE 看看有没有jdk开头的,没有的话就installed jres->add,选择java jdk安装目录,找到jdk的目录点击ok,Alternate JRE选择刚才添加的jdk
