Lecture - Imperative Programming TT23, XI
- 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?
finalize() called in Scala?Just before an object is garbage collected.