Java is pass by value. Think of it like a pointer language like C, the value of the pointer (memory address) is being passed, so you have a reference to the same object. Primitives aren't stored internally the same way as Objects, so when you pass a primitive's value, it's the content, not a pointer.
References might be implemented by storing the address. Usually Java references will be implemented as pointers, but that's not required by the specification. They may be using an additional layer of indirection to enable easier garbage collection. But in the end it will (almost always) boil down to (C-style) pointers being involved in the implementation of (Java-style) references.
You can't do pointer arithmetic with references. The most important difference between a pointer in C and a reference in Java is that you can't actually get to (and manipulate) the underlying value of a reference in Java. In other words: you can't do pointer arithmetic.
In C you can add something to a pointer (i.e. the address) or substract something to point to things that are "nearby" or point to places that are at any place.
In Java, a reference points to one thing and that thing only. You can make a variable hold a different reference, but you can't just ask it to point to "the thing after the original thing".
References are strongly typed. Another difference is that the type of a reference is much more strictly controlled in Java than the type of a pointer is in C. In C you can have an int* and cast it to a char* and just re-interpret the memory at that location. That re-interpretation doesn't work in Java: you can only interpret the object at the other end of the reference as something that it already is (i.e. you can cast a Object reference to String reference only if the object pointed to is actually a String).
Those differences make C pointers more powerful, but also more dangerous. Both of those possibilities (pointer arithmetic and re-interpreting the values being pointed to) add flexibility to C and are the source of some of the power of the language. But they are also big sources of problems, because if used incorrectly they can easily break assumptions that your code is built around. And it's pretty easy to use them incorrectly.
Best Answer
The Java Language Specification doesn't say anything about how big a reference is in memory. The JLS doesn't even say that a reference needs to exist at runtime at all. The compiler is free to use 42 gigabytes for a reference or to eliminate it completely, making the size 0 bytes. (In fact, the Java Language Specification never says anything about a particular implementation strategy or representation, it only specifies the syntax and semantics of Java's language constructs, not their pragmatics.)
Most typical Java Implementations will simply implement references as pointers, thus making them the same size as in C. Sometimes, these references are referred to as OOPs (ordinary object pointers). But the Oracle Hotspot JVM, for example, has a feature called CompressedOOPs, which can make some references smaller than C pointers on 64 bit systems.