A place of your own for any Java you want to try: no steps, no checks, nothing to follow. Change Main.java, add files, break things. Your files live in the folder ~/playground and stay in this browser.
Run (Ctrl+Enter, ⌘↵ on a Mac) compiles every .java file in the folder and runs Main.
The files panel adds, renames and deletes files and folders; a folder is a package. Ctrl+B (⌘B on a Mac) in the editor shows or hides it, Ctrl+P (⌘P) opens a file by name.
The Terminal tab is a Linux shell: cd ~/playground, then javac, java or anything else.
Reset code in the ⋯ menu puts the starter Main.java back and deletes every other file in the folder.