Access restriction: The constructor ‘JFrame(String)’ is not API (restriction on required library ‘C:\Program Files\Java\jre1.8.0_261\lib\rt.jar’)
错误原因:jre配置错误
解决方法:项目右键 - Build Path - Configure Build Path ;选择“Libraries”面板 ,然后双击"JRE System Library…";选择“JavaSE-1.7”就可以了;