Question: i know that java programs are first compiled and a bytecode is generated which is platform independent. But my question is why is this bytecode interpreted in the next stage and not compiled even though compilation is faster than interpretation in general??

Answer: 
You answered your own question. Byte code is platform independent. If the compiled code was executed then it would not work on every OS. This is what C does and it is why you have to have one version for every OS. 

As others have suggested, the JVM does actually compile the code using [JIT](http://en.wikipedia.org/wiki/Just-in-time_compilation). It is just not saved anywhere. Here is a nice quote to sum it up

> 
> In a bytecode-compiled system, source code is translated to an
>  intermediate representation known as bytecode. Bytecode is not the
>  machine code for any particular computer, and may be portable among
>  computer architectures. The bytecode may then be interpreted by, or
>  run on, a virtual machine. The JIT compiler reads the bytecodes in
>  many sections (or in full rarely) and compiles them interactively into
>  machine language so the program can run faster
> 
> 
>
The JVM uses **Just in time compilation** <http://en.wikipedia.org/wiki/Just-in-time_compilation>, so it's much faster than pure interpretation.