What functionality would I lose if I disable browser-based Java?

Can someone tell me what I would or would not experience and and is Java really necessary these day for browsing?

This is a fairly good question. What it boils down to is this: If you need Java in a browser, you will know so right away. If you don’t need Java—and are even unaware if you are even using it—chances are good you will never miss it and never bump into it again; the risk of a casual user tripping over a site that would truly need Java to function in 2015 is rare at best nowadays.

Just so you understand the history of Java and the web, Java is essentially a “black box” virtual machine that allows you to code in Java and then run that code on any system that can run Java. The concept was that Java would be a middle ground platform that could run on any machine; Windows, Macintosh, Linux, etc… So the Java plug-in simply allows you to run Java apps from within a web browser. This was appealing in the early days of the Internet due to lack of cross-platform compatibilities and other “rough edges” of the early Internet.

But in 2015, most of the “fancy” base-level functionality that Java provided in the past—cool graphics and effects and such—are now handled within the browser itself via CSS, HTML and JavaScript. Back in the late-1990s/early-2000s lots of creative sites used Java because native browsers could not handle the tricks one could only do in Java back then. Heck, Java even came bundled as a basic plugin in Netscape Navigator back in the day thanks to the commonly accepted functionality it provided.

Now where you might miss it in 2015 depends on what kinds of sites you access. For example, I know that many financial systems accessible online—such as personal tax systems, payroll gateways and other such systems—use complex Java applications to allow end users to get a richer experience with their financial tools. So if you are using your machine to access web-based financial applications like that, then you definitely need Java enabled. But in my experience, even in those cases, many such institutions are slowly moving their archaic Java-based systems to a more stable, non-Java based setup that takes better advantage of modern web browser functionality.

For example, a little over a year ago anyone using the browser-based screen sharing software/service GoToMeeting needed to have Java enabled in their browser and as well as on their system. But as explained in this support forum thread on their site they have now officially dropped the Java requirement in favor of their own non-Java-based software tool:

In the past we used to use Java to automate launching our software, we have since replaced that method with our own launcher. Since introducing our launcher we no longer use Java.

So my advice is if Java worries you as a potential security concern on a browser level, just disable it for now. I’m pretty confident you will not miss it. And if—in like 8 to 9 months—you need Java for some reason, deal with it then.

You will encounter Java on the web mostly in form of Java applets. These are applications which appear as box-shaped areas embedded in webpages. How your browser handles these when you don't have Java enabled depends on the browser, but most show it in form of a gray rectangle which prompts you in some way to install Java.

So when you encounter a website which requires Java for an applet, you will know.

You won't see many Java applets anyway. While they were a common sight during the web boom in the late 90s / early 2000s, they become quite rare nowadays, because the technology got displaced by Adobe Flash which now itself gets displaced by HTML5.