First of all, I prefer C / C++ to Java, however there are a number of advantages to Java.
1) Java is safer, because it runs it's own virtual machine, it is so much harder to crash a computer than with C.
2) Java is more flexible, because it runs on a virtual machine, you do not need to re-compile it to get it to run on a different OS etc, the different OS just runs a different virtual machine.
3) Jave has hundreads of libaries that come standard with it, for everything from GUI's to network protocols.
4) Java has an excelent documentation system, Java Doc, which is a standard documentation system
There are more advantages