How many bytes of memory does a java.util.Date object use?

Question

I need to store a large amount of dates (potentially large enough that the amount of heap space used is a concern so please, no lectures on premature optimization), and I\'m

Answer 1

^{As answered here too:}

Easiest way to answer this question is to look at the source code of java.util.Date.

It has only 2 non-static fields (Java 1.7.0_55):

private transient long fastTime;
private transient BaseCalendar.Date cdate;

long has a memory size of 8 bytes and cdate is an object reference which has a size of 4 bytes. So a total of 12 bytes.

If cdate would be instantiated, it could require additional bytes in the memory, but if you look at the constructors too, sometimes it won't even be touched, and in others it will be null-ed at the end of the constructor, so the final result is also 12 bytes.

This is just for creating a Date. If you call methods on the Date (for example Date.toString()), that will create and store an object into the cdate field which will not be cleared. So if you call certain methods on the Date, its memory usage will increase.

Note: Object references might be 64 bit long on 64-bit JVMs in which case memory usage would be 16 bytes.

Note #2: Also note that this is just the memory usage of the Date object itself. Most likely you will store its reference somewhere, e.g. in an array or list or a field in some other class which will require additional 4 bytes (or maybe 8 bytes on 64 bit JVMs).