Maybe it has bugs..

BitReader.java

package jp.segfault.io;

import java.io.EOFException;
import java.io.IOException;
import java.io.InputStream;

public class BitReader {

	private InputStream in;
	private long data;
	private int  size;
	private final byte[] b = new byte[8];
	
	public BitReader(InputStream in) {
		this.in = in;
	}
	
	public long get(int n) throws IOException {
		if(n > 64) {
			throw new IllegalArgumentException("n > 64 --> " + n);
		}
		long dest = 0;
		if(n > size) {
			dest = data << (n -= size);
			int need = (n + 7) / 8;
			data = readFully(need);
			size = need * 8;
		}
		if(n > 0) {
			dest |= data >>> (size -= n);
			data &= ~(-1L <<  size);
		}
		return dest;
	}
	
	private long readFully(int n) throws IOException {
		assert(n <= 8);
		if(in.read(b, 0, n) != n) {
			throw new EOFException();
		}
		long dest = 0;
		for(int i = 0; i < n; ++i) {
			dest <<= 8;
			dest  |= (long)(b[i] & 0xff);
		}
		System.err.println("readFully:"+ dest);
		return dest;
	}

	public void clear() {
		data = 0;
		size = 0;
	}
	
	@Override
	public String toString() {
		return "BitReader{data="+ Long.toBinaryString(data) +",size="+ size +"}";
	}
	
}

BitWriter.java

package jp.segfault.io;

import java.io.IOException;
import java.io.OutputStream;

public class BitWriter {

	private OutputStream out;
	private int remData;
	private int remSize; // < 8

	public BitWriter(OutputStream out) {
		this.out = out;
	}
	
	public void put(long data, int size) throws IOException {
		if(size > 64) {
			throw new IllegalArgumentException("n > 64 --> " + size);
		}
		long tmp = remData;
		data |= tmp << size;
		
		int div = (remSize + size) / 8;
		int mod = (remSize + size) % 8;
		if (mod != 0) {
			remData = (int)(data & ~(-1L << mod));
			data >>= mod;
			data  |= tmp << size - mod;
		}
		remSize = mod;
		while(div > 0) {
			out.write((byte)(data >> 8 * --div));
		}
	}
	
	public void clear() {
		remData = 0;
		remSize = 0;
	}
	
	public void flush() throws IOException {
		if(remSize > 0) {
			out.write((remData << 8 - remSize) & 0xff);
		}
		remData = 0;
		remSize = 0;
	}
	
}