Multiple Source Code files?

I’m pretty new to Java. My question is how do I put my main() in one source code file and the class that I am developing in a second source code file? My instructor has given us a class, and we need to create our own class and import the given class into our class we make. I am finding issues with the CodeChef java, because unlike an IDE like eclipse, there is no project, as well as no way to name the file. How can I import my given file into the second source code I’m developing in the CodeChef java IDE?