macOS Java Programming.

Try textmate or the free smultron.

You can also try Xcode. Not used textpad but had a quick look at the screenshots and it looks like you have the same basic layout in Xcode as in textpad, ie directory view of files on the left, editor in the main panel and so on. In Xcode you can setup a Java project, and add existing files from your project. When you add files you can either add groups for your files, or folder references. If you add folder references then the project tree view will behave like a directory tree view.

There is also an Organizer view in Xcode. This might work well for you as an alternative to creating an Xcode project. You can add directories to the organizer and then browse and open files in the editor. In this way you're using Xcode simply as an editor.

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I'm assuming you don't want the full blown power of an IDE because Eclipse and Netbeans will both run on the Mac. Both are free and it wouldn't hurt to be familiar with them as you'll be using either one after you graduate.

vim and emacs (or emacs.app, if you like). I have TextMate too, it's nice and pretty, but I find myself spending more time in vim.

For a full-blown Java IDE, I'd suggest Netbeans. It runs as well on Mac as on any other platform.