Welcome to the Ranch
Please use copy-and-paste to post the error messages, not screen capture. Click the top left icon on the command window, then edit, then mark, then highlight the text with the mouse, then enter to copy to clipboard. It is much easier to read.
When you compiled the file, with
javac HelloWorldApp.java
I presume you got no error messages? If the file compiles, nothing happens; you only get error messages if something goes wrong. Try the "dir" instruction to see where the .class file is.
If it is in your current directory, try the earlier suggestion of
java -cp . HelloWorldApp
Remember the java tool doesn't require extensions.
If that doesn't work, post the code you are compiling.
You should not normally set a system classpath, but some applications eg QuickTime, set a system classpath which can b*gg*r up your normal Java settings. If that is the case add .; to the beginning of the system classpath. You can have several .s in your classpath; I have tried it, and it doesn't seem to do any harm.