With respect, why would anyone want to learn C after learning Java. Of this group of languages C is the earliest and most basic - still usefull but only for certain types of programming, mainly for small process controllers.
C++ (C with Classes) is the Object Oriented development from C. This gives full philosophy and use of Objects and is the development into a more meaningful programming paradigm.
Java is the next step - all the beauty of C++ and Objects without the difficulties of handling pointers and dynamic memory allocation and deallocation that so often leads to memory leaks. Java looks after all this for you and because of this is in many ways easier to learn then C++.
Java is now the language of choice for both straight applications, browser applets and network programming. Java programs are portable and will run under all flavours if Unix, Linux, Mac OS and Windows without the need to recompile.
If you are starting to learn from scratch, why not start with the latest developed and supported language? All resources are free - compiler, online tuition and reference manuals, forums and chat rooms. You can get top class professional help from the teams at Sun Microsystems. This means that it is probably the best language to learn if you are not in a class-room situation where you may be limited by academic choices.
Whatever language you learn, it will take many, many months of hard work to become proficient and years to become professional. Learning for a few weeks over a summer break (presumable far from full-time and with other distractions) will only allow you to gain a rudimentary understanding of any modern language - maybe it will motivate you to go further - I hope so.
I have been programming and teaching since 1962 and I hope my opinions may be of use to you - but they are only my opinions
Alan