After installing Java, the java command can be executed normally, but the Javac command cannot be executed.
The most painful actual combat comprehension (it's dawn TAT): 06:12:39, March 6, 2019
1 Uninstall the current Java, delete all configured Java environment variables.
2 Get the value of the environment variable Path and cut it to Notepad. Check if there are Java-related variable values, and if there are, delete all Java-related variable values. Then save the remaining variable values into Path.
4 Restart the computer and reinstall the JDK.
5 Reset the JDK environment