Write it. Run it. Break it.
Real Java and real Python, running inside your browser — nothing to install and nothing to submit. Pick a starting point, change it, and see what happens. Breaking things on purpose is the fastest way to learn what the error messages mean.
Each language downloads a real runtime the first time you use it — a JVM for Java, CPython for Python. That takes a few seconds and needs a network connection. After that it stays loaded and runs quickly.
Java here is compiled by the Eclipse compiler on a Java 8 runtime,
while Ed uses javac on Java 23. Almost everything behaves identically,
but error wording can differ — Eclipse says
userName cannot be resolved to a variable where javac says
cannot find symbol. The very newest Java syntax will not run here.
Ed is what counts for your marks; this is for experimenting.
An infinite loop cannot be interrupted — reload the page if a program stops responding. Check your loop conditions before you run.