来自byte []的BitSet，长度很奇怪

Question

my code is :

String blah = "blah";
byte[] blahBytes = blah.getBytes("US-ASCII");
System.out.println(Arrays.toString(blahBytes));
BitSet set = BitSet.valueOf(blahBytes);
System.out.println(set.length());

the output is :

[98, 108, 97, 104]
31

Why is length() returning 31? Shouldn't it be 32?

Answer 1

Bit set length is determined by the position of the highest bit set to 1 . Since all bytes that you pass to construct bit set represent ASCII character subset of UNICODE, the 8-th bit is always zero. Therefore, the highest bit set to 1 will be either bit 30 or bit 31, depending on the letter or digit in the end of your string: if you pass "bla1" instead of "blah" you would get 30 ( demo 1 ). If you use control characters, such as <TAB> you could get an even shorter bit set of 28 ( demo 2 ).

If you would like to get a length rounded up to the next multiple of 8, use

int roundedLength = 8 * ((set.length() + 7) / 8);

demo 3