之前阅读了Short源码，今天来阅读下包装类Integer源码。
缓存
还记得Short源码中的缓存么？
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 （意思是我们可以指定缓存数据大小呢）
int h = 127;
String integerCacheHighPropValue =
sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
if (integerCacheHighPropValue != null) {
try {
int i = parseInt(integerCacheHighPropValue);
i = Math.max(i, 127);
// Maximum array size is Integer.MAX_VALUE （最大size integer.max_value）
h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
} catch( NumberFormatException nfe) {
// If the property cannot be parsed into an int, ignore it.
}
}
high = h;
cache = new Integer[(high - low) + 1];
int j = low;
for(int k = 0; k < cache.length; k++)
cache[k] = new Integer(j++);
// range [-128, 127] must be interned (JLS7 5.1.7)
// 断言 high 必须大于等于127
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}
看起来比Short的复杂一些，其实添加了可以指定缓存的功能。 想想为什么low一直都是-127 不开放指定呢？（猜测负数用的比较少，缓存负数没什么意义）
成员变量
public static final int BYTES = SIZE / Byte.SIZE; //4
@Native public static final int SIZE = 32; //这里有个元注解@Native英文不好看不懂是干什么的，short源码就没有添加
//用来辅助计算（String）value的长度 100->3
final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,
99999999, 999999999, Integer.MAX_VALUE };
final static char [] DigitOnes = { //这个和下面的使用挺有意思的
'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',
} ;
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'
};
@Native public static final int MAX_VALUE = 0x7fffffff;
@Native public static final int MIN_VALUE = 0x80000000;
构造函数
public Integer(int value) {
this.value = value;
}
public Integer(String s) throws NumberFormatException {
// short 中就是借助Integer的parseInt来进行解析，其实就是解析字符串
this.value = parseInt(s, 10);
}
//pareseInt 关键代码
......
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) {
// 这里等研究了Character再来解密
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;
......
toString
public String toString() {
return toString(value);
}
public static String toString(int i) {
if (i == Integer.MIN_VALUE)
return "-2147483648";
//负数多一个-号
int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i);
char[] buf = new char[size];
//这是一个比较有意思的方法
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); q*64+q*32+q*4=q*(64+32+4)=q*100
r = i - ((q << 6) + (q << 5) + (q << 2));
i = q;
//假如r=45,想想DigitOnes，DigitTens的设计
buf [--charPos] = DigitOnes[r];
buf [--charPos] = DigitTens[r];
}
// Fall thru to fast mode for smaller numbers
// assert(i <= 65536, i);
for (;;) {
//用乘法代替除法
q = (i * 52429) >>> (16+3); // 约等于 i*(2^20/10)/2^19
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;
}
}
valueOf
//可以传入字符串的进制，可传入的进制范围 2-36
public static Integer valueOf(String s, int radix) throws NumberFormatException {
return Integer.valueOf(parseInt(s,radix));
}
public static Integer valueOf(String s) throws NumberFormatException {
//默认10进制
return Integer.valueOf(parseInt(s, 10));
}
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
getInteger
//获取系统的参数
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 | NullPointerException e) {
}
if (v != null) {
try {
//解码，因为有各种进制
return Integer.decode(v);
} catch (NumberFormatException e) {
}
}
return val;
}
对比
public int compareTo(Integer anotherInteger) {
return compare(this.value, anotherInteger.value);
}
public static int compare(int x, int y) {
return (x < y) ? -1 : ((x == y) ? 0 : 1);
}
// MIN_VALUE = 0x80000000;
public static int compareUnsigned(int x, int y) {
return compare(x + MIN_VALUE, y + MIN_VALUE);
}
其他有意思的方法
//注意符号位
public static long toUnsignedLong(int x) {
return ((long) x) & 0xffff ffffL;
}
//返回无符号余数
public static int remainderUnsigned(int dividend, int divisor) {
// In lieu of tricky code, for now just use long arithmetic.
return (int)(toUnsignedLong(dividend) % toUnsignedLong(divisor));
}
//无符号除法
public static int divideUnsigned(int dividend, int divisor) {
// In lieu of tricky code, for now just use long arithmetic.
return (int)(toUnsignedLong(dividend) / toUnsignedLong(divisor));
}
//最高位1
public static int highestOneBit(int i) {
// HD, Figure 3-1 1001->1111 1111-0111 （二进制）
i |= (i >> 1);
i |= (i >> 2);
i |= (i >> 4);
i |= (i >> 8);
i |= (i >> 16);
return i - (i >>> 1);
}
//最低位1
public static int lowestOneBit(int i) {
// HD, Section 2-1
// 补码
return i & -i;
}
//返回最高位之前的 0
public static int numberOfLeadingZeros(int i) {
// HD, Figure 5-6 经典2分法，仔细想想十分巧妙
if (i == 0)
return 32;
int n = 1;
if (i >>> 16 == 0) { n += 16; i <<= 16; }
if (i >>> 24 == 0) { n += 8; i <<= 8; }
if (i >>> 28 == 0) { n += 4; i <<= 4; }
if (i >>> 30 == 0) { n += 2; i <<= 2; }
n -= i >>> 31;
return n;
}
//返回尾0个数
public static int numberOfTrailingZeros(int i) {
// HD, Figure 5-14
int y;
if (i == 0) return 32;
int n = 31;
y = i <<16; if (y != 0) { n = n -16; i = y; }
y = i << 8; if (y != 0) { n = n - 8; i = y; }
y = i << 4; if (y != 0) { n = n - 4; i = y; }
y = i << 2; if (y != 0) { n = n - 2; i = y; }
return n - ((i << 1) >>> 31);
}
//统计1的个数
public static int bitCount(int i) {
// HD, Figure 5-2
i = i - ((i >>> 1) & 0x55555555);
i = (i & 0x33333333) + ((i >>> 2) & 0x33333333);
i = (i + (i >>> 4)) & 0x0f0f0f0f;
i = i + (i >>> 8);
i = i + (i >>> 16);
return i & 0x3f;
}
