Java源码分析第3篇 - Java数值类型(二)

为了能够快速将数值转换为字符类型，程序采用了空间换时间的策略，预先使用数组存储了一些需要的数。

这段代码看的我火急火燎的，主要有几个问题没弄明白：

（1）为什么要对大于和小于65536的数要分开处理？

（2）q=(i*52429)>>>(16+3)不太看得懂

哪位大神给指点一下感激不尽~~~

好了，我们继续往下看，再一个主要的方法就是decode()了，源代码如下：

 public static Integer decode(String nm) throws NumberFormatException {
        int radix = 10;
        int index = 0;
        boolean negative = false;
        Integer result;

        if (nm.length() == 0)
            throw new NumberFormatException("Zero length string");
        char firstChar = nm.charAt(0);
        // Handle sign, if present
        if (firstChar == '-') {
            negative = true;
            index++;
        } else if (firstChar == '+')
            index++;

        // Handle radix specifier, if present
        if (nm.startsWith("0x", index) || nm.startsWith("0X", index)) {
            index += 2;
            radix = 16;
        }
        else if (nm.startsWith("#", index)) {
            index ++;
            radix = 16;
        }
        else if (nm.startsWith("0", index) && nm.length() > 1 + index) {
            index ++;
            radix = 8;
        }

        if (nm.startsWith("-", index) || nm.startsWith("+", index))
            throw new NumberFormatException("Sign character in wrong position");

        try {
            result = Integer.valueOf(nm.substring(index), radix);
            result = negative   Integer.valueOf(-result.intValue()) : result;
        } catch (NumberFormatException e) {
            // If number is Integer.MIN_VALUE, we'll end up here. The next line
            // handles this case, and causes any genuine format error to be
            // rethrown.
            String constant = negative   ("-" + nm.substring(index))
                                       : nm.substring(index);
            result = Integer.valueOf(constant, radix);
        }
        return result;
    }

如上函数的功能就是将字符串转换为整数，接受以八进制、十进制和16进制格式书写的字符串,测试如上方法：

System.out.println(Integer.decode("0x0011"));
System.out.println(Integer.decode("0X0011"));
System.out.println(Integer.decode("011"));
System.out.println(Integer.decode("-011"));

合法的字符串表示的数值最后都是以十进制的形式返回的，或者还可以使用parseInt()方法，源代码如下：

 public static int parseInt(String s, int radix)throws NumberFormatException{
        /*
         * WARNING: This method may be invoked early during VM initialization
         * before IntegerCache is initialized. Care must be taken to not use
         * the valueOf method.
         */

        if (s == null) {
            throw new NumberFormatException("null");
        }
        if (radix < Character.MIN_RADIX) {
            throw new NumberFormatException("radix " + radix +  " less than Character.MIN_RADIX");
        }
        if (radix > Character.MAX_RADIX) {
            throw new NumberFormatException("radix " + radix + " greater than Character.MAX_RADIX");
        }

        int result = 0;
        boolean negative = false;
        int i = 0, len = s.length();
        int limit = -Integer.MAX_VALUE;
        int multmin;
        int digit;

        if (len > 0) {
            char firstChar = s.charAt(0);
            if (firstChar < '0') { // Possible leading "+" or "-"
                if (firstChar == '-') {
                    negative = true;
                    limit = Integer.MIN_VALUE;
                } else if (firstChar != '+')
                    throw NumberFormatException.forInputString(s);

                if (len == 1) // Cannot have lone "+" or "-"
                    throw NumberFormatException.forInputString(s);
                i++;
            }
            multmin = limit / radix;
            while (i < len) {
                // Accumulating negatively avoids surprises near MAX_VALUE
                digit = Character.digit(s.charAt(i++),radix);
                if (digit < 0) {
                    throw NumberFormatException.forInputString(s);
                }
                if (result < multmin) {
                    throw NumberFormatException.forInputString(s);
                }
                result *= radix;
                if (result < limit + digit) {
                    throw NumberFormatException.forInputString(s);
                }
                result -= digit;
            }
        } else {
            throw NumberFormatException.forInputString(s);
        }
        
        return negative   result : -result;
    }

parseInt("0", 10) returns 0
parseInt("473", 10) returns 473
parseInt("+42", 10) returns 42
parseInt("-0", 10) returns 0
arseInt("-FF", 16) return