What is the difference between a variable, object, and reference?

Question

Exactly what are the differences between variables, objects, and references?

For example: they all point to some type, and they must all hold values (unless of course you have the temporary null-able type), but precisely how are their functions and implementations different from each other?

Example:

Dog myDog = new Dog(); //variable myDog that holds a reference to object Dog
int x = 12; //variable x that hold a value of 12

They have the same concepts, but how are they different?

Answer 1

(Just to be clear, the explanation I'm giving here is specific to Java and C#. Don't assume it applies to other languages, although bits of it may.)

I like to use an analogy of telling someone where I live. I might write my address on a piece of paper:

• A variable is like a piece of paper. It holds a value, but it isn't the value in itself. You can cross out whatever's there and write something else instead.
• The address that I write on the piece of paper is like a reference. It isn't my house, but it's a way of navigating to my house.
• My house itself is like an object. I can give out multiple references to the same object, but there's only one object.

Does that help?

The difference between a value type and a reference type is what gets written on the piece of paper. For example, here:

int x = 12;

is like having a piece of paper with the number 12 written on it directly. Whereas:

Dog myDog = new Dog();

doesn't write the Dog object contents itself on the piece of paper - it creates a new Dog, and then writes a reference to the dog on that paper.

In non-analogy terms:

• A variable represents a storage location in memory. It has a name by which you can refer to it at compile time, and at execution time it has a value, which will always be compatible with its compile-time type. (For example, if you've got a Button variable, the value will always be a reference to an object of type Button or some subclass - or the null reference.)
• An object is a sort of separate entity. Importantly, the value of a variable or any expression is never an object, only a reference. An object effectively consists of:
  • Fields (the state)
  • A type reference (can never change through the lifetime of the object)
  • A monitor (for synchronization)
• A reference is a value used to access an object - e.g. to call methods on it, access fields etc. You typically navigate the reference with the . operator. For example, if foo is a Person variable, foo.getAddress().getLength() would take the value of foo (a reference) and call getAddress() on the object that that reference refers to. The result might be a String reference... we then call getLength() on the object that that reference refers to.