源码看JAVA【五】Byte

1、定义常量，byte取值范围-128~127，位数为8位

    /**
     * A constant holding the minimum value a {@code byte} can
     * have, -27.
     */
    public static final byte   MIN_VALUE = -128;

    /**
     * A constant holding the maximum value a {@code byte} can
     * have, 27-1.
     */
    public static final byte   MAX_VALUE = 127;

    /**
     * The number of bits used to represent a {@code byte} value in two's
     * complement binary form.
     *
     * @since 1.5
     */
    public static final int SIZE = 8;

2、toString

      返回十进制数值

    public String toString() {
        return Integer.toString((int)value);
    }

3、hashCode

        byte的int值

    @Override
    public int hashCode() {
        return Byte.hashCode(value);
    }


    public static int hashCode(byte value) {
        return (int)value;
    }

4、valueOf

       byte只有256个值，初始化对象后放入缓存中，直接读取。

    private static class ByteCache {
        private ByteCache(){}

        static final Byte cache[] = new Byte[-(-128) + 127 + 1];

        static {
            for(int i = 0; i < cache.length; i++)
                cache[i] = new Byte((byte)(i - 128));
        }
    }

    public static Byte valueOf(byte b) {
        final int offset = 128;
        return ByteCache.cache[(int)b + offset];
    }

5、parseByte

      解析字符串时，以Integer进行解析成byte，超过byte取值范围则抛出异常

    public static byte parseByte(String s, int radix)
        throws NumberFormatException {
        int i = Integer.parseInt(s, radix);
        if (i < MIN_VALUE || i > MAX_VALUE)
            throw new NumberFormatException(
                "Value out of range. Value:\"" + s + "\" Radix:" + radix);
        return (byte)i;
    }


    public static byte parseByte(String s) throws NumberFormatException {
        return parseByte(s, 10);
    }

6、字符串构造函数

         以字符串十进制进行解析

    public Byte(String s) throws NumberFormatException {
        this.value = parseByte(s, 10);
    }