Question

I am currently designing a data structure in which I’m trying to keep memory consumption to a minimum. I have a few instance variables that may be null depending on the placement of the Node in the Trie. I started going down the path of creating separate classes (one that has the instance variable and one that doesn’t) so that I won’t waste tons of space with null references… but then I started to wonder how the jvm works. Does it still eat up the full 8 bytes (assuming x64 arch) if the object reference is null or does it have a more optimal way to store a null reference?

Answer 1

I’m almost sure JVMs will use as much space for a null reference as for any other one. If you have an object with say 3 reference fields, and you null out the one in the middle, I don’t think any virtual machine would be able to move the 3rd one and save the 4 or 8 bytes (and then when you changed the null to something else, it would have to move stuff around again). If if this was technically feasible, it would not be worth it – the extra computational cost and code complexity would kill any potential gains. Also, partly due to C heritage, on most machines a pointer equal bit-wise to 0 works as a NULL at a very low level, so the null reference has a rather obvious representation.

On Oracle/Sun JDK you can use -XX:+UseCompressedOops command line argument to make references 4 bytes instead of 8 on 64-bit if your heap is smaller than 16 GB (regardless of whether the reference is null or not).