Why Java is known as  platform independent, whats the internal mechanism which enables this.

Most popular Operating Systems are
1. Windows
2. Ubuntu
3. Mac

By definition  Operating System is a resource allocator(basic essence of OS ). The real difference comes in the way they allocate resources , the algorithms and optimization each has done to allocate the resources( hard disk, CPU , RAM , network).
Depending on the way of allocating resource the System calls to interact with the Operating Systems are also different for different Operating System. The instructions they need to execute a task is also different.

The executable file that a OS can understand varies across OS.

•  Windows : EXE file format
• Ubuntu    : ELF file format

For a program to become a process, the program needs to be present in the format the OS understands.

Lets Take the case of C language.

• A code written in C compiled in Windows will result in a executable which can never be understood by Ubuntu as the exe file format is different from ELF and vice versa.
• A C library if it needs to be shared across multiple OS platforms , then the code needs to be compiled against the different OS.
• Binary formed on one platform cannot be used on other.

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How does JAVA manages to be OS independent.

1.  Java code is compiled  to create a .class file format.
2.  The class file format is neither EXE or ELF its a different file format .
3.  The Class file is consumed by the Java Virtual Machine which in turn generates platform dependent instructions.
4.  Java Virtual Machine is a abstraction over the actual machine and provides its own instruction set hence its known as Virtual Machine.
5.  Java Virtual Machine is a binary and is machine dependent. For windows JVM is a EXE file, for Linux JVM a ELF file.
6.  Java Virtual Machine takes .class file as input and generates instruction pertaining to the platform.
7.  JVM code has been compiled against the Different OS and on a given platform , the JVM binary specific to the platform needs to be used on that platform.
8.  JVM input instruction set is constant across all the platforms , the OS specific instructions generated by the JVM is different for different platform.
9.  SUN JVM is written in C while the Java Compiler is written in JAVA.

On Ubuntu JVM file Type is

file /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java
/usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=8b0e245cdb52ffc478774ec7c54577531ecc9394, stripped