java中字节与位的转换

java中byte与bit之间的转换

使用场景：用于解析自定义协议，比如某一位代表一个状态

public class ByteUtils {

    /**
     * 获取一个字节的bit数组
     *
     * @param value
     * @return
     */
    public static byte[] getByteArray(byte value) {
        byte[] byteArr = new byte[8]; //一个字节八位
        for (int i = 7; i > 0; i--) {
            byteArr[i] = (byte) (value & 1); //获取最低位
            value = (byte) (value >> 1); //每次右移一位
        }
        return byteArr;
    }

    /**
     * 把byte转为字符串的bit
     *
     * @param b
     * @return
     */
    public static String byteToBitString(byte b) {
        return ""
                + (byte) ((b >> 7) & 0x1) + (byte) ((b >> 6) & 0x1)
                + (byte) ((b >> 5) & 0x1) + (byte) ((b >> 4) & 0x1)
                + (byte) ((b >> 3) & 0x1) + (byte) ((b >> 2) & 0x1)
                + (byte) ((b >> 1) & 0x1) + (byte) ((b >> 0) & 0x1);
    }

    /**
     * 获取一个字节第n位,
     * 思路：右移n位，与1
     *
     * @param value
     * @param index
     * @return
     */
    public static int get(byte value, int index) {
        return (value >> index) & 0x1;
    }

    /**
     * 获取一个字节的第m到第n位
     *
     * @param value
     * @param start >0
     * @param end   >0
     * @return
     */
    public static byte[] getBitRange(byte value, int start, int end) {
        byte[] rangeArray = new byte[end - start + 1];
        if (start > 7 || start < 0) {
            throw new RuntimeException("illegal start param");
        }
        if (end > 7 || end < 0) {
            throw new RuntimeException("illegal end param");
        }
        if (start > end) {
            throw new RuntimeException("start can not bigger than end");
        }
        if (start == end) {
            rangeArray[0] = (byte) ByteUtils.get(value, start);
            return rangeArray;
        }
        for (int i = end; i < start; i--) {
            rangeArray[i] = (byte) ByteUtils.get(value, start);
        }
        return rangeArray;
    }

    /**
     * 位字符串转字节
     * @param str
     * @return
     */
    public static byte bitStringToByte(String str) {
        if(null == str){
            throw new RuntimeException("when bit string convert to byte, Object can not be null!");
        }
        if (8 != str.length()){
            throw new RuntimeException("bit string'length must be 8");
        }
        try{
            //判断最高位，决定正负
            if(str.charAt(0) == '0'){
                return (byte) Integer.parseInt(str,2);
            }else if(str.charAt(0) == '1'){
                return (byte) (Integer.parseInt(str,2) - 256);
            }
        }catch (NumberFormatException e){
            throw new RuntimeException("bit string convert to byte failed, byte String must only include 0 and 1!");
        }

        return 0;
    }