Integer源码分析

Integer中包含了大量的static方法。

1.分析Integer的缓存机制：首先定义了一个缓存区，IntegerCache，其实就是一个Integer数组cache[]，它默认存储了从-128~127这些Integer对象。

 private static class IntegerCache {
         static final int low = -128;
         static final int high;
         static final Integer cache[];

         static {
             // high value may be configured by property
             int h = 127;
             String integerCacheHighPropValue =
                 sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
             if (integerCacheHighPropValue != null) {
                 int i = parseInt(integerCacheHighPropValue);
                 i = Math.max(i, 127);
                 // Maximum array size is Integer.MAX_VALUE
                 h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
             }
             high = h;

             cache = new Integer[(high - low) + 1];
             int j = low;
             for(int k = 0; k < cache.length; k++)
                 cache[k] = new Integer(j++);
         }

         private IntegerCache() {}
     }

调用该内部类的代码是valueOf方法，

在这里的assert IntegerCache.high >= 127 我认为是用于加载IntegerCache并执行static初始化代码段使用的（好像也不对，因为后面if语句中也可以初始化，???????请大神指正）。

 public static Integer valueOf(int i) {
         assert IntegerCache.high >= 127;
         if (i >= IntegerCache.low && i <= IntegerCache.high)
             return IntegerCache.cache[i + (-IntegerCache.low)];
         return new Integer(i);
     }

2.getInteger方法用于获取对应系统字符串代表的整形数，如果想要直接获取字符串代表的整形数，使用decode()方法，如decode(“123”) ----> 123

 public static Integer getInteger(String nm) {
         return getInteger(nm, null);
     }

 public static Integer getInteger(String nm, int val) {
         Integer result = getInteger(nm, null);
         return (result == null) ? Integer.valueOf(val) : result;
     }

 public static Integer getInteger(String nm, Integer val) {
         String v = null;
         try {
             v = System.getProperty(nm);
         } catch (IllegalArgumentException e) {
         } catch (NullPointerException e) {
         }
         if (v != null) {
             try {
                 return Integer.decode(v);
             } catch (NumberFormatException e) {
             }
         }
         return val;
     }

　　getInteger方法最终调用decode方法返回整数,decode方法如下：其实最终调用的还是方法parseInt，这里只是将字符串的前缀判断一下，数字的部分用parseInt方法获取结果

 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;
     }

3.parseInt方法，该方法将为纯数字的字符串以规定的进制radix（2~36，查看Character类可得到）转为相应的int数字

 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;
     }

 public static int parseInt(String s) throws NumberFormatException {
         return parseInt(s,10);
     }

4.静态的toString方法：源代码的实现方式比较奇怪，先贴在这里：

//一个代表整形的串中可能出现的字符 所组成的数组
final static char[] digits = {
        '0' , '1' , '2' , '3' , '4' , '5' ,
        '6' , '7' , '8' , '9' , 'a' , 'b' ,
        'c' , 'd' , 'e' , 'f' , 'g' , 'h' ,
        'i' , 'j' , 'k' , 'l' , 'm' , 'n' ,
        'o' , 'p' , 'q' , 'r' , 's' , 't' ,
        'u' , 'v' , 'w' , 'x' , 'y' , 'z'
    };

//将整形以对应的基数radix转为字符串
public static String toString(int i, int radix) {

        if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX)
            radix = 10;

        /* Use the faster version */
        if (radix == 10) {
            return toString(i);
        }

        char buf[] = new char[33];
        boolean negative = (i < 0);
        int charPos = 32;

        if (!negative) {
            i = -i;
        }

        while (i <= -radix) {
            buf[charPos--] = digits[-(i % radix)];
            i = i / radix;
        }
        buf[charPos] = digits[-i];

        if (negative) {
            buf[--charPos] = '-';
        }

        return new String(buf, charPos, (33 - charPos));
    }

public static String toHexString(int i) {
        return toUnsignedString(i, 4);
    }

public static String toOctalString(int i) {
        return toUnsignedString(i, 3);
    }

public static String toBinaryString(int i) {
        return toUnsignedString(i, 1);
    }

//shift表示将基数左移的位数，然后
 private static String toUnsignedString(int i, int shift) {
        char[] buf = new char[32];
        int charPos = 32;
        int radix = 1 << shift;
        int mask = radix - 1;
        do {
            buf[--charPos] = digits[i & mask];
            i >>>= shift;
        } while (i != 0);

        return new String(buf, charPos, (32 - charPos));
    }

 final static char [] DigitTens = {
        '0', '0', '0', '0', '0', '0', '0', '0', '0', '0',
        '1', '1', '1', '1', '1', '1', '1', '1', '1', '1',
        '2', '2', '2', '2', '2', '2', '2', '2', '2', '2',
        '3', '3', '3', '3', '3', '3', '3', '3', '3', '3',
        '4', '4', '4', '4', '4', '4', '4', '4', '4', '4',
        '5', '5', '5', '5', '5', '5', '5', '5', '5', '5',
        '6', '6', '6', '6', '6', '6', '6', '6', '6', '6',
        '7', '7', '7', '7', '7', '7', '7', '7', '7', '7',
        '8', '8', '8', '8', '8', '8', '8', '8', '8', '8',
        '9', '9', '9', '9', '9', '9', '9', '9', '9', '9',
        } ;

    final static char [] DigitOnes = {
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
        } ;

public static String toString(int i) {
        if (i == Integer.MIN_VALUE)
            return "-2147483648";
        int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i);
        char[] buf = new char[size];
        getChars(i, size, buf);
        return new String(buf, true);
    }

static void getChars(int i, int index, char[] buf) {
        int q, r;
        int charPos = index;
        char sign = 0;

        if (i < 0) {
            sign = '-';
            i = -i;
        }

        // Generate two digits per iteration
        while (i >= 65536) {
            q = i / 100;
        // really: r = i - (q * 100);
            r = i - ((q << 6) + (q << 5) + (q << 2));
            i = q;
            buf [--charPos] = DigitOnes[r];
            buf [--charPos] = DigitTens[r];
        }

        // Fall thru to fast mode for smaller numbers
        // assert(i <= 65536, i);
        for (;;) {
            q = (i * 52429) >>> (16+3);
            r = i - ((q << 3) + (q << 1));  // r = i-(q*10) ...
            buf [--charPos] = digits [r];
            i = q;
            if (i == 0) break;
        }
        if (sign != 0) {
            buf [--charPos] = sign;
        }
    }

    final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,
                                      99999999, 999999999, Integer.MAX_VALUE };

    // Requires positive x
    static int stringSize(int x) {
        for (int i=0; ; i++)
            if (x <= sizeTable[i])
                return i+1;
    }

后续可以参考：http://www.cnblogs.com/hanmou/p/3463984.html