If you write a piece of Java ...

Java automatically works both within a web page and stand-alone.

You have to write separate pieces of Java if you want to run them in both a web page and stand-alone, although many

If you're careful, the same piece of Java can be used both as a stand-alone program and within a web page.

You run your Java first in a web page. A button then lets you save a local copy which you can run stand-alone