今天写完代码运行时，出现了程序安装失败的对话框：
Installation failed since the device possibly has stale dexed jars that don't match the current version (dexopt error). In order to proceed, you have to uninstall the existing application.
WARNING: Uninstalling will remove the application data!
Do you want to uninstall the existing application?
已运行就是这样，死活没办法，最后搞了一晚上，终于找到了答案，AndroidStudio-> Build -> Clean Projects 然后等下面任务完成后，重新运行就可以了。