Java Hello appears with the following result: Bad command or the file name may be the cause

No such command or file name

This may be due to a failure to install the JDK successfully or an environment variable that does not have a JDK configured, or to compile the appropriate file.

2.

The following result appears: Exception in thread "main" Java.lang.NoClassDefFoundError:Hello

What could be the reason

There may be no class defined, directly written into Main's bytecode.

3.

Input command: Java hello.class, what will happen, why?

The main class could not be found or could not be loaded, Java Hello is already loading the class, no need to add to the back

Java Hello appears with the following result: Bad command or the file name may be the cause