Why is wrong bit representation got by converting byte array to BitSet in Java

Question

I tried to convert a byte array in order to use its bit representation.

Input:

byte[] values = new byte[]{21, 117, 41, -1};

I would like to create one BitSet object from the byte array but I splitted up it to investigate the issue and tried to create multiple BitSet objects from each element of the array.

BitSet objects are created on the following way:

BitSet bitSetTwentyOne = BitSet.valueOf(new byte[]{21});
BitSet bitSetOneHundredSevenTeen = BitSet.valueOf(new byte[{117});
BitSet bitSetFourtyOne = BitSet.valueOf(new byte[]{41});
BitSet bitSetMinusOne = BitSet.valueOf(new byte[]{-1});

The bits are printed out by using the following method:

private String getBits(BitSet bitSet) {
    StringBuilder bitsBuilder = new StringBuilder();

    for (int i = 0; i < bitSet.length(); i++) {
        bitsBuilder.append(bitSet.get(i) == true ? 1 : 0);
    }

    return bitsBuilder.toString();
}

Output:

bitSetTwentyOne: 10101 but expected -> 00010101 (BitSet::length = 5)
bitSetOneHundredSevenTeen: 1010111 but expected -> 01110101 (BitSet::length = 7)
bitSetFourtyOne: 100101 but expected -> 00101001 (BitSet::length = 6)
bitSetMinusOne: 11111111 but it is as expected at least (BitSet::length = 8)

I want all values 8bit width even if it is needed to fill with zeros. I do not understand why it gives wrong binary values in case of converting 117 and 41 .

Answer 1

• length() delivers the logical length using the highest bit 1.
• size() would give 8.
• cardinality() would give the number of bit 1s.

So you should have used size() .

And then the bits are in little endian format, and you reversed it by outputting bit at 0 first.

private String getBits(BitSet bitSet) {
    StringBuilder bitsBuilder = new StringBuilder();
    for (int i = 0; i < bitSet.size(); ++i) {
        bitsBuilder.insert(0, bitSet.get(i) ? '1' : '0');
    }
    return bitsBuilder.toString();
}

Answer 2

It appears that you are printing the bits backwards. Notice how your output (except for the desired leading zeros) is a mirror image of your expected input.

Using your code, passing in 4 , which should be 100 , I get 001 . It appears that bit 0 is the least significant bit, not the most significant bit. To correct this, loop over the indices backwards. The reason that -1 and 41 are correct apart from the leading zeros is that their bitset representations are palindromes.

To prepend leading zeros, subtract the length() from 8 and print that many zeros. The length() method only returns the number of bits necessary to represent the number. You need to code this yourself; there is no BitSet functionality for leading zeros.