Is it possible to read/write bits from a file usin

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 possible?

I want to use it for a compression algorithm, so when I am compressing I want to write 3 bits(for one number and there are millions of such numbers in a file) and if I write a byte at everytime I need to write 3 bits, I will write loads of redundant data...

Answer 1

Afaik there is no function for doing this in the Java API. However you can of course read a byte and then use bit manipulation functions. Same goes for writing.

Answer 2

If you are just writing bits to a file, Java's BitSet class might be worth a look at. From the javadoc:

This class implements a vector of bits that grows as needed. Each component of the bit set has a boolean value. The bits of a BitSet are indexed by nonnegative integers. Individual indexed bits can be examined, set, or cleared. One BitSet may be used to modify the contents of another BitSet through logical AND, logical inclusive OR, and logical exclusive OR operations.

You are able to convert BitSets to long[] and byte[] to save data to a file.

Answer 3

Moved to https://github.com/jinahya/bit-io

Please take a look at http://jinahya.googlecode.com/svn/trunk/com.googlecode.jinahya/bit-io/src/main/java/com/googlecode/jinahya/io/

<dependency>
  <!-- resides in central repo -->
  <groupId>com.googlecode.jinahya</groupId>
  <artifactId>bit-io</artifactId>
  <version>1.0-alpha-13</version>
</dependency>

This is a small handy library for reading/writing arbitrary length of bits with Java.

final InputStream stream;
final BitInput input = new BitInput(new BitInput.StreamInput(stream));

final int b = input.readBoolean(); // reads a 1-bit boolean value
final int i = input.readUnsignedInt(3); // reads a 3-bit unsigned int
final long l = input.readLong(47); // reads a 47-bit signed long

input.align(1); // 8-bit byte align; padding


final WritableByteChannel channel;
final BitOutput output = new BitOutput(new BitOutput.ChannelOutput(channel));

output.writeBoolean(true); // writes a 1-bit boolean value
output.writeInt(17, 0x00); // writes a 17-bit signed int
output.writeUnsignedLong(54, 0x00L); // writes a 54-bit unsigned long

output.align(4); // 32-bit byte align; discarding