Java UUID's long representation

Question

I'm storing tons of Java UUID into a HashMap as row using UUID.toString(). Since the data is huge, soon it throws OutOfMemoryError. Now I'm thinking about a compact way to represent the UUID, preferably something like long, and then later I can easily reconstruct the UUID with that long representation. Is this possible?

Answer 1

I'm storing tons of Java UUID into a HashMap as row using UUID.toString().

So by this you mean HashMap<String, MyObject>?

Storing UUID itself via HashMap<UUID, MyObject> will conserve space compared to HashMap<String, MyObject>. A UUID takes up less space than the String does (two long values become 16 bytes versus char[36] for 72 bytes, that'll save you almost 80% space right there).

If changing that's not enough, then consider whether or not the UUID values are important across JVM's. If the ID only needs to be unique for a single process (are you saving the HashMap to disk or sharing it between Java processes?), then you can just use int since a HashMap can't be any bigger than Integer.MAX_VALUE anyway. So instead of HashMap<UUID, MyObject>, you'd have HashMap<Integer, MyObject>. Even better, you can use Short if you are going to have less than 2¹⁶ objects, saving even more space. However, if you're getting an OutOfMemoryError, I suspect you probably have more than 65536 objects.

Lastly, if all else fails, allocate more memory to your JVM as demonstrated in this question.

Answer 2

A UUID is fundamentally a number, but it's a 128-bit number, which is twice the size of a java long. You could use BigInteger (which is probably no more space-efficient than storing UUIDs as strings), or you could encapsulate the UUID in an object that contains two longs — one for the first 64 bits and one for the last 64 bits.

Given the UUID 550e8400-e29b-41d4-a716-446655440000, you would want to create two longs, one containing the number 0x550e8400e29b41d4, and one containing the number 0xa716446655440000.