why operating systems are not written in java ?

In theory, yes. But you would need to somehow get the Java VM running using low-level code (unless you want to compile Java down to assembly, which probably isn't possible without sacrificing a lot of Java's features).

In short, Java need to run on JVM. JVM need to run on an OS. writing an OS using Java is not a good choice.

The Need of Hardware Interpreted compiler

 OS needs to deal with hardware which is not doable using java (except using JNI). And that is because JVM only provided limited commands which can be used in Java. These command including add, call a method and so on. But deal with hardware need command to operate reg, memory, CPU, hardware drivers directly. These are not supported directly in JVM so JNI is needed. That is back to the start - it is still needed to write an OS using C/assembly.

Although it is theoretically possible to write some OS in pure java, there are practical matters that make this task really difficult. The main problem is that there is no (currently up to date and reliable) java compiler able to compile java to byte code. So there is no existing tool to make writing a whole OS from the ground up feasible in java, at least as far as my knowledge goes.

The Magic of Java Virtual Machine Makes more advantages

 Java was designed to run in some implementation of the java virtual machine. There exist implementations for Windows, Mac, Linux, Android, etc. The design of the language is strongly based on the assumption that the JVM exists and will do some magic for you at runtime (think garbage collection, JIT compiler, reflection, etc.). This is most likely part of the reason why such a compiler does not exist: where would all these functionality go? Compiled down to byte code? It's possible but at this point I believe it would be difficult to do. Even Android, whose SDK is purely java based, runs Dalvik (a version of the JVM that supports a subset of the language) on a Linux Kernel.

Source:http://stackoverflow.com/questions/29580170/why-operating-systems-are-not-written-in-java