PATH has nothing to do with finding .java files.
PATH is just to do with executable files.. (and possibly others if adding certain extensions to PATHEXT, but that use case isn't so relevant to your problem, and anyhow PATH wouldn't have anything to do with specifying a file as a parameter to a program, which is what you were expecting)
You have to either go to the path where the file is and do javac helloworld.java
Or, you specify the path of the java file when you run javac.
There is such a thing as CLASSPATH, but that is for .class files and java.exe Not for javac and .java files.
One use of PATH would be for finding javac.exe and java.exe (Running them from wherever you are) , but not for locating .java files. It looks like you already have that sorted, certainly for javac.exe anyway. Your PATH as it is for this.