Byte 类源码分析

说明：Byte 类的实现，有一部分方法是借助于 Integer 类的实现.

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* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package jdk.java.lang;

/**
*
* The {@code Byte} class wraps a value of primitive type {@code byte}
* in an object.  An object of type {@code Byte} contains a single
* field whose type is {@code byte}.
*
* <p>In addition, this class provides several methods for converting
* a {@code byte} to a {@code String} and a {@code String} to a {@code
* byte}, as well as other constants and methods useful when dealing
* with a {@code byte}.
*
* @author  Nakul Saraiya
* @author  Joseph D. Darcy
* @see     java.lang.Number
* @since   JDK1.1
*/
public final class Byte extends Number implements Comparable<Byte> {

    /**
     * byte 最小值的常量{@code byte}可以具有-2 ^ 7.
     * Java 中用补码表示二进制数, 补码最高位表示符号位，0 表示正数，1 表示负数.
     * 正数补码为其本身; 负数补码为其各位取反加1；
     */
    public static final byte   MIN_VALUE = -128;

    /**
     * byte 最大值为 2 ^ 7 -1.
     */
    public static final byte   MAX_VALUE = 127;

    /**
     * The {@code Class} instance representing the primitive type
     * {@code byte}.
     */
    public static final Class<Byte>     TYPE = (Class<Byte>) Class.getPrimitiveClass("byte");

    /**
     * 返回表示指定的{@code byte}的新{@code String}对象. 假设基数为10.
     *
     * @param b the {@code byte} to be converted
     * @return the string representation of the specified {@code byte}
     * @see java.lang.Integer#toString(int)
     */
    public static String toString(byte b) {
        return Integer.toString((int)b, 10);
    }

    /**
     * Byte对象的缓存类
     */
    private static class ByteCache {
        private ByteCache(){}

        /**
         * 缓存数组
         */
        static final Byte cache[] = new Byte[-(-128) + 127 + 1];

        static {
            // 缓存初始化，范围是-128到127
            for(int i = 0; i < cache.length; i++)
                cache[i] = new Byte((byte)(i - 128));
        }
    }

    /**
     * 封装基础byte到Byte对象，不过就是从缓存数组中取出对应的值.
     * 说明：这里的这种处理方法和 Boolean 中的 valueOf 方法的处理方式类似，都是先定义
     * 好了取值范围，然后直接返回定义好的值的引用即可.
     *
     * @param  b a byte value.
     * @return a {@code Byte} instance representing {@code b}.
     * @since  1.5
     */
    public static Byte valueOf(byte b) {
        final int offset = 128;
        return ByteCache.cache[(int)b + offset];
    }

    /**
     * 将字符串按照指定的基数 radix 解析成 byte.
     *
     * @param s         the {@code String} containing the
     *                  {@code byte}
     *                  representation to be parsed
     * @param radix     the radix to be used while parsing {@code s}
     * @return          the {@code byte} value represented by the string
     *                   argument in the specified radix
     * @throws          NumberFormatException If the string does
     *                  not contain a parsable {@code byte}.
     */
    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;
    }

    /**
     * 将字符串 s 解析成 基数为 10 进制的 byte.
     *
     * @param s         a {@code String} containing the
     *                  {@code byte} representation to be parsed
     * @return          the {@code byte} value represented by the
     *                  argument in decimal
     * @throws          NumberFormatException if the string does not
     *                  contain a parsable {@code byte}.
     */
    public static byte parseByte(String s) throws NumberFormatException {
        return parseByte(s, 10);
    }

    /**
     * 将字符串按照指定的基数 radix 解析成 Byte.
     *
     * @param s         the string to be parsed
     * @param radix     the radix to be used in interpreting {@code s}
     * @return          a {@code Byte} object holding the value
     *                  represented by the string argument in the
     *                  specified radix.
     * @throws          NumberFormatException If the {@code String} does
     *                  not contain a parsable {@code byte}.
     */
    public static Byte valueOf(String s, int radix)
        throws NumberFormatException {
        return valueOf(parseByte(s, radix));
    }

    /**
     * 将字符串 s 解析成 基数为 10 进制的 Byte.
     *
     * @param s         the string to be parsed
     * @return          a {@code Byte} object holding the value
     *                  represented by the string argument
     * @throws          NumberFormatException If the {@code String} does
     *                  not contain a parsable {@code byte}.
     */
    public static Byte valueOf(String s) throws NumberFormatException {
        return valueOf(s, 10);
    }

    /**
     * 将{@code String}解码为{@code Byte}. 接受以下语法给出的十进制，十六进制和八进制数：
     * 0x|0X|#|0
     *
     * @param     nm the {@code String} to decode.
     * @return   a {@code Byte} object holding the {@code byte}
     *          value represented by {@code nm}
     * @throws  NumberFormatException  if the {@code String} does not
     *            contain a parsable {@code byte}.
     * @see java.lang.Byte#parseByte(java.lang.String, int)
     */
    public static Byte decode(String nm) throws NumberFormatException {
        int i = Integer.decode(nm);
        if (i < MIN_VALUE || i > MAX_VALUE)
            throw new NumberFormatException(
                    "Value " + i + " out of range from input " + nm);
        return valueOf((byte)i);
    }

    /**
     * The value of the {@code Byte}.
     *
     * @serial
     */
    private final byte value;

    /**
     * 构造一个新分配的{@code Byte}对象，该对象表示指定的{@code byte}值.
     *
     * @param value     the value to be represented by the
     *                  {@code Byte}.
     */
    public Byte(byte value) {
        this.value = value;
    }

    /**
     * 构造一个新分配的{@code Byte}对象，该对象表示{@code String}参数指示的{@code byte}值.
     * 字符串将以{@code parseByte}方法用于基数10的方式转换为{@code byte}值.
     *
     * @param s         the {@code String} to be converted to a
     *                  {@code Byte}
     * @throws           NumberFormatException If the {@code String}
     *                  does not contain a parsable {@code byte}.
     * @see        java.lang.Byte#parseByte(java.lang.String, int)
     */
    public Byte(String s) throws NumberFormatException {
        this.value = parseByte(s, 10);
    }

    /**
     * Returns the value of this {@code Byte} as a
     * {@code byte}.
     */
    public byte byteValue() {
        return value;
    }

    /**
     * Returns the value of this {@code Byte} as a
     * {@code short}.
     */
    public short shortValue() {
        return (short)value;
    }

    /**
     * Returns the value of this {@code Byte} as an
     * {@code int}.
     */
    public int intValue() {
        return (int)value;
    }

    /**
     * Returns the value of this {@code Byte} as a
     * {@code long}.
     */
    public long longValue() {
        return (long)value;
    }

    /**
     * Returns the value of this {@code Byte} as a
     * {@code float}.
     */
    public float floatValue() {
        return (float)value;
    }

    /**
     * Returns the value of this {@code Byte} as a
     * {@code double}.
     */
    public double doubleValue() {
        return (double)value;
    }

    /**
     * Returns a {@code String} object representing this
     * {@code Byte}'s value.  The value is converted to signed
     * decimal representation and returned as a string, exactly as if
     * the {@code byte} value were given as an argument to the
     * {@link java.lang.Byte#toString(byte)} method.
     *
     * @return  a string representation of the value of this object in
     *          base 10.
     */
    public String toString() {
        return Integer.toString((int)value);
    }

    /**
     * Returns a hash code for this {@code Byte}; equal to the result
     * of invoking {@code intValue()}.
     *
     * @return a hash code value for this {@code Byte}
     */
    public int hashCode() {
        return (int)value;
    }

    /**
     * Compares this object to the specified object.  The result is
     * {@code true} if and only if the argument is not
     * {@code null} and is a {@code Byte} object that
     * contains the same {@code byte} value as this object.
     *
     * @param obj       the object to compare with
     * @return          {@code true} if the objects are the same;
     *                  {@code false} otherwise.
     */
    public boolean equals(Object obj) {
        if (obj instanceof Byte) {
            return value == ((Byte)obj).byteValue();
        }
        return false;
    }

    /**
     * Compares two {@code Byte} objects numerically.
     *
     * @param   anotherByte   the {@code Byte} to be compared.
     * @return  the value {@code 0} if this {@code Byte} is
     *          equal to the argument {@code Byte}; a value less than
     *          {@code 0} if this {@code Byte} is numerically less
     *          than the argument {@code Byte}; and a value greater than
     *           {@code 0} if this {@code Byte} is numerically
     *           greater than the argument {@code Byte} (signed
     *           comparison).
     * @since   1.2
     */
    public int compareTo(Byte anotherByte) {
        return compare(this.value, anotherByte.value);
    }

    /**
     * 以数字方式比较两个{@code byte}值.
     *
     * <pre>
     *    Byte.valueOf(x).compareTo(Byte.valueOf(y))
     * </pre>
     *
     * @param  x the first {@code byte} to compare
     * @param  y the second {@code byte} to compare
     * @return the value {@code 0} if {@code x == y};
     *         a value less than {@code 0} if {@code x < y}; and
     *         a value greater than {@code 0} if {@code x > y}
     * @since 1.7
     */
    public static int compare(byte x, byte y) {
        return x - y;
    }

    /**
     * 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;

    /** use serialVersionUID from JDK 1.1. for interoperability */
    private static final long serialVersionUID = -7183698231559129828L;
}