Is it possible to read/write bits from a file using JAVA?

Question

To read/write binary files, I am using DataInputStream/DataOutputStream, they have this method writeByte()/readByte(), but what I want to do is read/write bits? Is it possib

Answer 1

The below code should work

    int[] mynumbers = {3,4};
    BitSet compressedNumbers = new BitSet(mynumbers.length*3);
    // let's say you encoded 3 as 101 and 4 as 010
    String myNumbersAsBinaryString = "101010"; 
    for (int i = 0; i < myNumbersAsBinaryString.length(); i++) {
        if(myNumbersAsBinaryString.charAt(i) == '1')
            compressedNumbers.set(i);
    }
    String path = Resources.getResource("myfile.out").getPath();
    ObjectOutputStream outputStream = null;
    try {
        outputStream = new ObjectOutputStream(new FileOutputStream(path));
        outputStream.writeObject(compressedNumbers);
    } catch (IOException e) {
        e.printStackTrace();
    }

Answer 2

It's not possible to read/write individual bits directly, the smallest unit you can read/write is a byte.

You can use the standard bitwise operators to manipulate a byte though, so e.g. to get the lowest 2 bits of a byte, you'd do

byte b = in.readByte();
byte lowBits = b&0x3;

set the low 4 bits to 1, and write the byte:

b |= 0xf;
out.writeByte(b);

(Note, for the sake of efficiency you might want to read/write byte arrays and not single bytes)

Answer 3

Bits are packaged in bytes and apart from VHDL/Verilog I have seen no language that allows you to append individual bits to a stream. Cache up your bits and pack them into a byte for a write using a buffer and bitmasking. Do the reverse for read, i.e. keep a pointer in your buffer and increment it as you return individually masked bits.