How to give each object in a document a unique ID?

I'm making a bitmap editor where a document consists of several layers where each layer represents a bitmap. Each layer must have a unique ID compared to all other layers that currently exist in the document. I also need to take into account that I need to save and load documents along with the layer IDs.

I'm using the command pattern to store actions that are performed on the document and the uniqu开发者_StackOverflowe IDs are used to keep track of which layer an action should be performed on.

At the moment, I just keep a counter called X, when a new layer is created its ID is set to X then X is incremented. When loading, I need to make sure X is set to an appropriate number so that new layers are given unique IDs i.e. I could save the value of X and restore that, or set X based on the biggest layer ID loaded.

Given X is a 32-bit number, the user will need to create 4,294,967,296 layers working on the same file before IDs start to be reused which will cause weird behaviour. Should I implement a better unique ID system or is this generally good enough?

I'm in Java so I could use the UUID library which creates 128 bit unique IDs according to a standard algorithm. This seems overkill though.

Is there some general approach to this kind of problem?

This is perfectly good enough. At a rate of ten new layers per second 24/365, which is silly, it'll run fine for roughly three years.

If you think you might manipulate layers programmatically, and thus have some possibility of having 2^32 layers in the lifetime of the image, then throw all the layer IDs into a HashSet when you read the file, and update that set when you add/remove layers. (You don't need to explicitly store the set; the ID associated with each mask is enough.) Then instead of taking "the next number", take "the next number not in the set". Having a counter is still useful, but whether you set it to 0 or (max+1) when you read a file is immaterial; it won't take a significant amount of time to find an empty space unless you envision bafflingly large numbers of layers present simultaneously.

But since you're using Java, you could just use a long instead of an int, and then you wouldn't ever (practically speaking) be able to overflow the number even if all you did was create a one-pixel mask and destroy it over and over again.