I wanted to upload some .java files on github, I don't remember if I uploaded the right files. I have been told that the files I should've submitted were missing. I checked the last commit, and the files I should've submitted were marked with "BIN" and were of type "classname".class instead of "classname".java, underneath each one of them it says "Binary file not shown" instead of the code. I would like to know what happened so that I can avoid this in the future. I have never experienced this before on github. What did I upload? I don't think I had any .class files regarding this project.
From what you say seems like you added files to your commit that shouldn't be added.
Well, to fix this, just remove those files, commit and push the changes. Done.
To avoid this scenario in the future and not only for you but for your team, you can create a .gitignore file at the root of the repository. This file contains patterns of paths (files and folders) so git will ignore those files when showing the state of the branch. If there's no such .gitignore file then create one and add the following:
*.class
bin/
Also, you can add more entries in this file to support omitting other files generated by IDE, for example. There are curated lists you can find like https://gist.github.com/chhh/4961200 or https://www.gitignore.io/api/eclipse