To better grasp the idea of reference types, let’s look at the other kind of type: value types. While reference-type variables refer to a place in memory, value-type variables hold the actual data.
int is a value type, so the variable num holds the value 6:
Reference types, on the other hand, refer to a location in memory. Every class is a reference type, so the variable diss refers to a location in memory that has the Dissertation object:
Every “primitive” data type is a value type, including:
intdoubleboolchar
Revisiting our metaphor: a reference is like directions to a house, which “points” to a house. It isn’t the actual house. A value type is the house itself!
You might have noticed that string is missing here. It works a bit differently, so it will be covered in a later lesson.
Instructions
The diagram to the right represents a computer’s memory:
- An object is stored in the first memory block
diss1hold a reference to the first memory blockdiss2also holds a reference to the first memory blocknumrefers to a value in the fourth memory block
Notice that the object takes up more memory than either reference (size is represented by the width of the slot), and that changing the object would affect both diss1 and diss2.