One of Microsoft's best friends just hit the gas pedal on its already huge growth

Xamarin, a fast-growing mobile app development startup that's got a close partnership with Microsoft, has snapped up tiny startup RoboVM in a little acquisition that could lead to some huge growth.

In the world of business app development, two programming languages stand above the rest: C# (pronounced "C-Sharp"), the most recent standard for writing Microsoft Windows software. And Java, the for just about everything else.

When Xamarin first got started in 2011, it had a simple sales pitch. Write your smartphone app in C#, and it provides the tools to make it into an iPhone, Android, Mac, or Windows app with a minimum of effort.

RoboVM, a tiny startup founded earlier in 2015, has the exact same pitch - only it did it with Java.

"Now we can get the other half," says Xamarin co-founder and CEO Nat Friedman.

By buying RoboVM, Xamarin gets access to the tremendous Java developer market, which analyst firm IDC pegs at between 5 to 7 million programmers worldwide. It's the software development equivalent of flooring the gas pedal.

Growing like crazy

Not that Xamarin needed to increase its base, Friedman says.

Xamarin announced recently that it's adding 50,000 users a day, with over a million registered developers. It specializes in helping enterprises build their mobile apps, which is a lucrative market as businesses move to software in droves. And Friedman now says that Xamarin has over 10,000 paying customers.

"We're not hurting for growth," says Friedman. "The amount of software being created to solve business problems is way, way higher than it's ever been."

With the addition of Java developers to their addressable market, it simply "removes a filter" and lets Xamarin serve more customers. It used to be that when Xamarin went to a customer, they'd have to ask how many C# developers were on staff, since that would affect how much value they'd get from it.

But thanks to Java's popularity in the enterprise , it's a pretty safe bet that if they're not using C#, they're using Java.

Everybody <3's Xamarin

Developers love the Xamarin concept, since it drastically cuts down the time it takes to make sure their apps work on every smartphone platform out there. It also helps developers build in the features that are specifically designed for whatever device their app is running on, like cameras and NFC readers.

"Many apps get abandoned if the apps suck," Friedman says.

Microsoft likes Xamarin a lot, too, because it millions of existing Windows developers on board with C# while letting them take advantage of the growth in other platforms.

In fact, that relationship with Windows developers is what led Xamarin to raising an $54 million round of financing last year. Rumors persist that Microsoft was actually an unnamed investor in that round.

Plus, Xamarin has deals with Oracle and IBM to help bring apps to their cloud platforms, too.

Now, with Java and RoboVM, it's still the same Xamarin. But it's one that has a much broader appeal.