In Spain it used to be a Java desktop app as well, the last incarnation of a software that started as a DOS program distributed in diskettes back in 1988. Now it's no longer a standalone program, but a website (not sure which technology, but it runs in current browsers without requiring Java installed, AFAIR).

The government makes a draft declaration for you, if it's OK you just have to digitally sign your agreement and provide a bank account to pay or receive your return (if you haven't already done that in the past). Otherwise you need to edit it adding whatever they've missed, but this is only needed if you do complex stock trading, operate with money abroad, etc. For most people it's just skimming through it in case there is an obvious error, and confirming the draft.

Once you confirm the draft, it takes only around 2-4 days for the tax return money to appear in your bank account.

The website's UI is good. Spanish e-administration webapps are a bit hit-and-miss, some are dreadful (digital certificate request, I'm looking at you), but that particular one is fine and has always worked well for me.