java源码学习
在51CTO上看java视频的时候，看到了parseInt这个函数，是Integer的方法，
看到了valueOf这个函数，是String的方法，
int num = 100;
//String s = String(num);
String s = String.valueOf(num);
在String.java中：
public static String valueOf(int i) {
return Integer.toString(i);
}
Integer.java:
//Integer.java
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 int stringSize(int x) {
for (int i=0; ; i++)
if (x <= sizeTable[i])
return i+1;//进位
}
static void getChars(int i, int index, char[] buf) {
int q, r;
int charPos = index;//index <-size
char sign = 0;
if (i < 0) {
sign = '-';
i = -i;
}
// Fall thru to fast mode for smaller numbers
// assert(i <= 65536, i);
for (;;) {
q = (i * 52429) >>> (16+3);//q = i*(52429/2^16)/2^3 = 52429/524288 ?= 0.1//2^19
r = i - ((q << 3) + (q << 1)); // r = i-(q*10) ...
buf [--charPos] = digits [r];//逆序存入数组
i = q;
if (i == 0) break;
}
if (sign != 0) {
buf [--charPos] = sign;
}
}
为了提高效率，使用了查表(空间换时间)和位运算
用到的数组
//Integer.java
final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,
99999999, 999999999, Integer.MAX_VALUE };
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',
} ;
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',
} ;