Q:

Can i combine many jar files in one jar file

I have multiple JAR files, which I have to add to classpath in Eclipse.
Is it possible to combine 30 files in one file and include that file?

A:

You can, but I don't think it would necessarily be a good idea to do so. Three possible reasons, and no doubt there are more:

It makes it harder to see where any one constituent file comes from.
It makes it harder to replace just one library
If there are files which are contained in multiple jar files, which should "win"? For example, all the jar files will have their own manifest... you may be able to merge them all, but not necessarily... and for other files there may well simply not be a sensible way of merging them.

My vote is to keep the jar files separate.

A:

Your may want to have a look at jarjar.
If you use an ant task you can also go for zipgroupfileset:
<zip destfile="jarlibrary.jar">
    <zipgroupfileset dir="lib" includes="*.jar"/>
</zip>

A:

There are a number of existing tools for this:  

Uberjar
Megajar
Onejar

