Most people think that JRE (Java Runtime Environment) and .NET are almost similar. But there is a major difference between those two approaches.
.NET – Write from different languages, compile to an intermediate state which can be run on Common Language Runtime (CLR). You can run programs written in C#, J#, C++,F# and many more on the .NET framework.
Java and JRE(Java Runtime Environment) – Write code in Java, run on any platform on top of JRE. That is, you can run any program written in Java on any operating system in which JRE is installed.
Therefore you can see that java programs can be run on any platform since JRE is currently available for all the major OS platforms. But .NET is not available for Linux platform directly yet (can be run on top of WINE). As a design, Microsoft has put a step forward than Java, by allowing multiple languages to be run on top of .NET framework. But it is not as same as JRE since .NET framework is not widely available for all the other platforms except windows. But there is Mono. Mono is the .NET port for Linux/MacOS. It is available on Ubuntu & Mint Linux.