How do I get the number of bits represented by a BitSet?

问题

If I run the following code:

BitSet test = new BitSet(20);
System.out.println("Bitset size is " + test.size());
System.out.println("Bitset length is " + test.length());

I get the output:

Bitset size is 64
Bitset length is 0

Which makes sense now that I look closer at the documentation (the former getting the size including implementation overhead and the latter being the last set bit, with all defaulting to false), but is not what I want.

Since the BitSet I actually use can have varying lengths, I want to be able to get back the number of bits represented (ignoring whether they are set). Is there a way to do that (hopefully with a built-in method)?

I realize I could try flipping all the bits and then doing length, or by tracking the variable I use to instantiate the BitSet, but I feel like both would require several lines of comments with my reasons and I was hoping for something a bit more self-documenting.

回答1:

A BitSet will automatically expand to a size sufficient to represent the highest bit set in it; the initial size is simply a hint as to how many bits you intend to use. So your question as posed makes no sense.

回答2:

I did some testing on my Mac and it appears that the BitSet length wants to be a multiple of 64. So, for your BitSet size of 20, it defaulted to 64. If you set your size to 65, then you should see a length of 128. Setting it to 129 should give you 192. This should give you a way to calculate your 'true' BitSet length.

回答3:

Bitset internally uses long (64 bits). So you can determine number of bits needed in multiples of this using the size() method.

来源：https://stackoverflow.com/questions/8371585/how-do-i-get-the-number-of-bits-represented-by-a-bitset