在實現Huffman樹時，壓縮和解壓需要從檔案中按位讀取，即一次只讀一個位或者寫一個位。
《演算法（第四版）》中提供了一種實現方法，StdIn類和StdOut類，豁然開朗。

按位讀取

方法：
1. 開闢一個緩衝區int buffer，用於儲存位，並記錄緩衝區中bit數n
2. 緩衝區不為空時，進行位運算buffer>>(n-1)，即為buffer中的最高位，實現讀取一個位的操作

例:

buffer向右移7個位buffer >> (8-1)，即可得到最高位1

public
 
 static boolean readBoolean() {
    if (isEmpty()) throw new NoSuchElementException("Reading from empty input stream");
    n--;
    boolean bit = ((buffer >> n) & 1) == 1;
    if (n == 0) fillBuffer();
    return bit;
}

其他部分實現

public final class BinaryStdIn {
    private static BufferedInputStream in = new
 
 BufferedInputStream(System.in);
    private static final int EOF = -1;    // end of file

    private static int buffer;            // one character buffer
    private static int n;                 // number of bits left in buffer

    // static initializer
    static {
        fillBuffer();
    }

    // don't instantiate
 

    private BinaryStdIn() { }

    private static void fillBuffer() {
        try {
            buffer = in.read();
            n = 8;
        }
        catch (IOException e) {
            System.out.println("EOF");
            buffer = EOF;
            n = -1;
        }
    }

    /**
     * Returns true if standard input is empty.
     * @return true if and only if standard input is empty
     */
    public static boolean isEmpty() {
        return buffer == EOF;
    }
}

按位寫入

按位寫入的實現，主要是利用與運算|的特性：
1. 0 | n = n
2. 1 | n = 1

與按位讀取相同，我們同樣要使用一個緩衝區。

假設要寫入的位為 bit，則
1. 將buffer左移一位buffer <<= 1，空出一位存放bit
2. 如果bit為1，則 buffer = buffer | 1；如果bit為0，則buffer即為寫入bit後的緩衝區

例如：

程式碼實現

private static void writeBit(boolean bit) {
    // add bit to buffer
    buffer <<= 1;
    if (bit) buffer |= 1;
    // if buffer is full (8 bits), write out as a single byte
    n++;
    if (n == 8) clearBuffer();
} 

完整實現

public final class BinaryStdOut {
    private static BufferedOutputStream out = new BufferedOutputStream(System.out);

    private static int buffer;     // 8-bit buffer of bits to write out
    private static int n;          // number of bits remaining in buffer

    // don't instantiate
    private BinaryStdOut() { }

   /**
     * Write the specified bit to standard output.
     */
    private static void writeBit(boolean bit) {
        // add bit to buffer
        buffer <<= 1;
        if (bit) buffer |= 1;

        // if buffer is full (8 bits), write out as a single byte
        n++;
        if (n == 8) clearBuffer();
    } 

    private static void clearBuffer() {
        if (n == 0) return;
        if (n > 0) buffer <<= (8 - n);
        try {
            out.write(buffer);
        }
        catch (IOException e) {
            e.printStackTrace();
        }
        n = 0;
        buffer = 0;
    }