10.1java原始碼解析-Integer (1)
阿新 • • 發佈:2018-12-09
1類的宣告
public final class Integer extends Number implements Comparable<Integer>
- 繼承 Number
- 實現 Comparable<Integer> 可以比較大小
2類屬性
@Native public static final int MIN_VALUE = 0x80000000; @Native public static final int MAX_VALUE = 0x7fffffff; @Native public static final int SIZE = 32; public static final int BYTES = SIZE / Byte.SIZE;
3建構函式
public Integer(int value) {
this.value = value;
}
public Integer(String s) throws NumberFormatException {
this.value = parseInt(s, 10);
}
下面會對 parseInt(s, 10);進行詳細說明
4方法
4.1toString(int i, int radix)
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) {/*--------1*/ return toString(i); } char buf[] = new char[33];/*--------2*/ boolean negative = (i < 0); int charPos = 32; if (!negative) { i = -i; } while (i <= -radix) {/*--------3*/ buf[charPos--] = digits[-(i % radix)]; i = i / radix; } buf[charPos] = digits[-i]; if (negative) {/*--------4*/ buf[--charPos] = '-'; } return new String(buf, charPos, (33 - charPos)); }
- 1處的意思是如果是十進位制就直接返回字串
- 2處33字元陣列是因為 進位制最小是2進位制,而整型的2進位制是32位,再加上1一個符號位,所以就33個字元。
- 3處 進行進位制處理
- 4 新增符號
4.2toUnsignedString
獲取整型的補碼以 radix顯示
public static String toUnsignedString(int i, int radix) {
return Long.toUnsignedString(toUnsignedLong(i), radix);
}
補碼標註
- 正數的補碼還是原碼
- 負數的補碼是原碼取反加1。
4.3 進位制轉化
public static String toHexString(int i) {
return toUnsignedString0(i, 4);
}
public static String toOctalString(int i) {
return toUnsignedString0(i, 3);
}
public static String toBinaryString(int i) {
return toUnsignedString0(i, 1);/*-----1*/
}
private static String toUnsignedString0(int val, int shift) {
// assert shift > 0 && shift <=5 : "Illegal shift value";
int mag = Integer.SIZE - Integer.numberOfLeadingZeros(val);
int chars = Math.max(((mag + (shift - 1)) / shift), 1);
char[] buf = new char[chars]; /*-----2*/
formatUnsignedInt(val, shift, buf, 0, chars);/*-----3*/
// Use special constructor which takes over "buf".
return new String(buf, true);
}
static int formatUnsignedInt(int val, int shift, char[] buf, int offset, int len) {
int charPos = len;
int radix = 1 << shift;/*-----4*/
int mask = radix - 1;
do {
buf[offset + --charPos] = Integer.digits[val & mask];/*-----5*/
val >>>= shift;/*-----6*/
} while (val != 0 && charPos > 0);
return charPos;
}
通過原碼可以看出,轉化進位制呼叫的是同一個方法,只是引數不一樣
- 1呼叫相同方法
- 2通過上面程式碼操作,可以獲得最後轉化為進位制補碼的字串長度,從而減少不必要的記憶體消耗
- 3 呼叫求補碼的方法,引數
- val 原值
- shift 1代表2進位制,2代表4進位制,3代表8進位制,4代表16進位制
- buf 代表轉化完的進位制字串儲存處。
- offset 開始位置
- len 長度
- 4 將傳的 1,2,3,4 轉化為進位制數 2,4,8,16
- 5 將值與進位制數進行與運算,類似於 取餘操作,例如 例如20的2進製表示為 10100,10100&1111 =100 這就會獲得最後四位的2進位制碼。
- 6進行移位,把剛才處理過的位去掉。10100移位就是 1,然後再重複5操作,便會得到 14,而20的16進位制就是14。 通俗解釋
就是每 shift位數轉化為進位制數,也就是上問中的 1,2,3,4
4.4 toString(int i)
這個方法本來不想寫的,但看了原始碼,發現很多疑惑,分享一下
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'
};
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);/*-----1*/
char[] buf = new char[size];
getChars(i, size, buf);
return new String(buf);
}
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);/*-----2*/
r = i - ((q << 3) + (q << 1)); // r = i-(q*10) .../*-----3*/
buf [--charPos] = digits [r]; /*-----4*/
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;
}
- 想不到吧,tostring方法竟然這麼麻煩,我的疑惑是,上面既然已經有toString(int i,int j)的方法,為什麼不直接呼叫,為什麼tostring不直接 return ""+i; 呢
- 具體解釋如下
- 1 處的意思是 獲取 數的位數,如,12 的位數是2,2343的位數是4,檢視stringSize可知,是通過呼叫靜態陣列列表來獲取的。
- 2 處的意思是 個人理解,就是獲取最低位前所有位的數,例如,123獲取的就是12,1245獲取的就是124
- 3處的意思是 就是獲取最末位的數字
- 4 處的意思是根據數字大小來獲取字串中位置的值,並插入到char陣列中
4.5parseInt
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);
}
- 其實就是把字串拆解,然後每個char 都進行 Character.digit(s.charAt(i++),radix);解析,然後再拼接成整型
- 這裡有一個問題 ,按理說 parseInt(String s,int i),i應該表示的是進位制,但是,心在只有輸入10是對的,其他值都會異常
4.6parseUnsignedInt(String s, int radix)
public static int parseUnsignedInt(String s, int radix)
throws NumberFormatException {
if (s == null) {
throw new NumberFormatException("null");
}
int len = s.length();
if (len > 0) {
char firstChar = s.charAt(0);
if (firstChar == '-') {
throw new
NumberFormatException(String.format("Illegal leading minus sign " +
"on unsigned string %s.", s));
} else {
if (len <= 5 || // Integer.MAX_VALUE in Character.MAX_RADIX is 6 digits
(radix == 10 && len <= 9) ) { // Integer.MAX_VALUE in base 10 is 10 digits
return parseInt(s, radix);
} else {
long ell = Long.parseLong(s, radix);
if ((ell & 0xffff_ffff_0000_0000L) == 0) {
return (int) ell;
} else {
throw new
NumberFormatException(String.format("String value %s exceeds " +
"range of unsigned int.", s));
}
}
}
} else {
throw NumberFormatException.forInputString(s);
}
}