Bits.java

package java.io;


/**
 * Utility methods for packing/unpacking primitive values in/out of byte arrays
 * using big-endian byte ordering.
 */
class Bits {


    /*
     * Methods for unpacking primitive values from byte arrays starting at
     * given offsets.
     */


    static boolean getBoolean(byte[] b, int off) {
        return b[off] != 0;
    }


    static char getChar(byte[] b, int off) {
        return (char) ((b[off + 1] & 0xFF) +
                       (b[off] << 8));/*从offer 开始,取出一个char,byte是8位，若对char,byte,short进行移位，自动转换为int，b[off]长度为32位，移位不会溢出。大端：高位数在低地址*/
    }


    static short getShort(byte[] b, int off) {
        return (short) ((b[off + 1] & 0xFF) +
                        (b[off] << 8));
    }


    static int getInt(byte[] b, int off) { //需要截取4个byte，按大端法构成一个int
        return ((b[off + 3] & 0xFF)      ) +
               ((b[off + 2] & 0xFF) <<  8) +
               ((b[off + 1] & 0xFF) << 16) +
               ((b[off    ]       ) << 24);
    }


    static float getFloat(byte[] b, int off) {
        return Float.intBitsToFloat(getInt(b, off));
    }


    static long getLong(byte[] b, int off) {
        return ((b[off + 7] & 0xFFL)      ) +
               ((b[off + 6] & 0xFFL) <<  8) +
               ((b[off + 5] & 0xFFL) << 16) +
               ((b[off + 4] & 0xFFL) << 24) +
               ((b[off + 3] & 0xFFL) << 32) +
               ((b[off + 2] & 0xFFL) << 40) +
               ((b[off + 1] & 0xFFL) << 48) +
               (((long) b[off])      << 56);
    }


    static double getDouble(byte[] b, int off) {
        return Double.longBitsToDouble(getLong(b, off));
    }


    /*
     * Methods for packing primitive values into byte arrays starting at given
     * offsets.
     */


    static void putBoolean(byte[] b, int off, boolean val) {
        b[off] = (byte) (val ? 1 : 0);
    }


    static void putChar(byte[] b, int off, char val) {
        b[off + 1] = (byte) (val      ); //将char转换成byte数组，按大端法：char（16位）的高8位应该在b[off]，低8位在b[off+1]中。>>>是无符号右移
        b[off    ] = (byte) (val >>> 8);
    }


    static void putShort(byte[] b, int off, short val) {
        b[off + 1] = (byte) (val      );
        b[off    ] = (byte) (val >>> 8);
    }


    static void putInt(byte[] b, int off, int val) {
        b[off + 3] = (byte) (val       );
        b[off + 2] = (byte) (val >>>  8);
        b[off + 1] = (byte) (val >>> 16);
        b[off    ] = (byte) (val >>> 24);
    }


    static void putFloat(byte[] b, int off, float val) {
        putInt(b, off,  Float.floatToIntBits(val));
    }


    static void putLong(byte[] b, int off, long val) {
        b[off + 7] = (byte) (val       );
        b[off + 6] = (byte) (val >>>  8);
        b[off + 5] = (byte) (val >>> 16);
        b[off + 4] = (byte) (val >>> 24);
        b[off + 3] = (byte) (val >>> 32);
        b[off + 2] = (byte) (val >>> 40);
        b[off + 1] = (byte) (val >>> 48);
        b[off    ] = (byte) (val >>> 56);
    }


    static void putDouble(byte[] b, int off, double val) {
        putLong(b, off, Double.doubleToLongBits(val));
    }
}