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。