# Lecture - Imperative Programming TT23, XI

> Source: https://ollybritton.com/notes/uni/prelims/tt23/ip/lectures/xi/ · Updated: 2023-05-19 · Tags: uni, lecture

- Reference counting 
	- Associate a counter with each object
	- Increase when attaching to a reference
	- Decrease when detaching from a reference
	- If count is zero, then can garbage collect
	- Reference counting cannot deal with cyclic references
	- The solution is to free memory held by “unreachable code”, this can be done with a technique called “mark and sweep”
- Internalisation
	- An optimisation technique used for efficiently using memory when you don’t care about object identity
	- Create a hash map that contains already existing “equal” instances of a given class and use those instead
	- Can lead to memory leaks
	- Weak reference: like a normal reference but doesn’t affect if an object will be garbage collected
	- These can be used to solve memory leaks when using internalisation.
- Finalisers
	- `finalize()` is called before an object is garbage collected, e.g. could be used for closing files
	- Better to handle this explictly in the code, like adding a specific method for releasing resources

### Flashcards
When does reference counting increase or decrease the count associated with each object?::
- Increase, attaching to a reference
- Decrease, deatching from a reference

In what situation does reference counting fail, and what technique can be used to fix this?::
When there are cyclic references, using a technique called “mark and sweep”.

What is the “internalisation” technique?::
An optimisation technique used for efficieciently using memory, where you create a hashmap of objects that will “act” the same and reuse them.

What is a weak reference?::
A reference that doesn’t affect if an object will be garbage collected, might suddenly point to null.

When is `finalize()` called in Scala?::
Just before an object is garbage collected.

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