Java applets - is it a wrong choice today?

I thinks the biggest disadvantage of an applet is that it assumes you have a JRE installed on a client machine. Is it really a viable assumption?

Of course, you can offer to download and install JRE as well, but why bother doing all this only for making some computation? Another question I would ask myself, can your clients be mobile phones, tablets and so on? If so, maybe JavaScript is a better option to go with.

And yet another two cents :) You mentioned 'opened to eye JavaScript' You should understand that the only real way of protecting your computation code is putting the computation on server. I mean, that even if you have a compiled binary code, Java's assembly is easy-to-understand for a skilled attacker. And obfuscation that you mentioned (it's obfuscation, not signing a JAR file) makes it slightly harder, but still not impossible.

The only concern I see here is that if you have a lot of clients that are running the computation simultaneously and you put the burden of computation on your server it can collapse eventually.

As of September 2015 they are dead to me. There are pros and cons for using applets. But Chrome stopped supporting them, so by using them you simply don't support Chrome for desktop and when it comes to mobile browsers, which of them support NPAPI?

Official Oracle announcement:

Chrome no longer supports NPAPI (technology required for Java applets) The Java plug-in for web browsers relies on the cross platform plugin architecture NPAPI, which has been supported by all major web browsers for over a decade. Google's Chrome version 45 (scheduled for release in September 2015) drops support for NPAPI, impacting plugins for Silverlight, Java, Facebook Video and other similar NPAPI based plugins.

Java applications are offered through web browsers as either a web start application (which do not interact with the browser once they are launched) or as a Java applet (which might interact with the browser). This change does not affect Web Start applications, it only impacts applets.

If you have problems accessing Java applications using Chrome, Oracle recommends using Internet Explorer (Windows) or Safari (Mac OS X) instead.

UPDATE 1 Microsoft Edge also does not support them. So another blow against the already dying Java applets.

UPDATE 2

Announcement from Mozilla

Important: The new 64-bit version of Firefox for Windows does not recognize or support this plugin. See this Mozilla blog post for details.

So yeah. Java applets are dead.

UPDATE 3 Oracle officially killed them with Java 9.

UPDATE 4 Java Web Start is also dead. From Java 11