What is the difference between C and java?

You're comparing Apples and Oranges. C is a procedural language. You can, using structures and functions to access those structures do everything you can in Java. In fact, if you look into GObject, the object oriented method of programming in GTK (the Linux toolkit) people do this all the time. In fact they have pretty much implemented C++ manually using a programming style and macros.

C++ and Java have a great deal more in common. From a language perspective, they are nearly identical. Java is arguably cleaner as it doesn't depend on forward declaration and therefore all functions are coded directly within the class definitions themselves. C++ is arguably more powerful as you have direct access to the hardware of the system and it's also possible to write code in such a way that it is guaranteed to be compiled an optimized in a specific manor.

For application development, Java is often faster (performance and in the time it takes to get the job done). For system level development, C++ is almost always a clear winner. I won't detail it as it's far beyond the scope of the question.

Java

There are two key differences between Java and C++ to a programmer.

1) Java is a garbage collected language

This means you can allocate memory and just forget about it when you're done. Some people say this leads to ugly code and quite frankly, I agree. Bad programmers can be REALLY bad programmers in Java and get away with it. C++ a C++ program will tend to crash your machine if you write it badly. The selling point of garbage collection is performance. It allows the runtime system of the language to clean up when there's time to do it instead of having to clean up during time critical moments. In C++ this type of behavior has to be coded manually and requires a great deal of strategic planning to accomplish.

2) Java has a "good" defined set of standard class libraries.

C++ has libraries also, but only a small percentage of programmers use them because many of us (me included) consider them to be a plague on the language. They're VERY over-engineered and can be very hard to debug. Java's libraries are a lot cleaner even if they have serious inconsistencies (this may spur a political debate, but I offer my opinion and the arguments I've heard from others).

Neither language is particularly good. Neither runtime is particularly modern. There are newer technologies such as C#/.NET and others which have arguably learn from C++ and Java and built upon what was learned.

If you're learning to program, I recommend using C++ with Qt (from Nokia) as it gives you almost the best of both worlds. If you're using one and looking to switch to the other. It should be a smooth transition, the languages are basically the same.

If you're asking about internal data structure. The two languages aren't comparable as Java doesn't have a strict internal structure definition and therefore can't be compared.

P.S. - For a bonus, Java's best feature is relocatable memory. Because it forces you to never use memory locations directly (pointers), so the Java virtual machine can relocate your memory in order to make room for larger allocations that otherwise wouldn't fit without paging or asking the OS for more RAM. This can make your applications more efficient as virtual memory paging is far more expensive than simply defragging the memory occasionally.