JDK源码分析：Integer.java部分源码解析
1）声明部：
public final class Integer extends Number implements Comparable<Integer>
extends Number, 重写方法：
public byte byteValue() {
return (byte)value;
}
public short shortValue() {
return (short)value;
}
public int intValue() {
return value;
}
public long longValue() {
return (long)value;
}
public float floatValue() {
return (float)value;
}
public double doubleValue() {
return (double)value;
}
implements Comparable<T>，接口实现如下：
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);
}
与实现该接口无关观察compareUnsigned方法：
compareUnsigned 例子：
Integer a1 = 0b00000000000000000000000000000001;
Integer a2 = 0b00000000000000000000000000000011;
Integer a3 = 0b10000000000000000000000000000001;
Integer a4 = 0b10000000000000000000000000000011;
int r1 = Integer.compareUnsigned(a1.intValue(),a2.intValue());
int r2 = Integer.compareUnsigned(a1.intValue(),a3.intValue());
int r3 = Integer.compareUnsigned(a3.intValue(),a4.intValue());
Out.println("a1,a2,a3,a4分别为：" + a1 +"," + a2 + "," + a3 + "," + a4);
Out.println("r1,r2,r3分别为：" + r1 +"," + r2 + "," + r3);
result：
a1,a2,a3,a4分别为：1,3,-2147483647,-2147483645 r1,r2,r3分别为：-1,-1,-1
算法分析：比较是无符号比较方法，而默认是有符号整型，所以需要特殊方法处理。先看Integer的范围：
0B10000000000000000000000000000000
0B10000000000000000000000000000001
..
0B11111111111111111111111111111111
...
0B00000000000000000000000000000000
0B00000000000000000000000000000001
0B00000000000000000000000000000010
...
0B01111111111111111111111111111111
在同一个符号下：除符号位其他位的数字越大，那么该数字越大。+MIN.VALUE就是修改符号位。不影响同一个符号位下的大小比较,
假设a=-1,b=1按无符号考虑：a>b;
按有符合考虑：a<b，通过修改符号位，a+MIN.VALUE(正数)>b+MIN.VALUE(负数)
实现了无符号比较。
2）属性
@Native public static final int MIN_VALUE = 0x80000000;
@Native public static final int MAX_VALUE = 0x7fffffff;
/**
* The {@code Class} instance representing the primitive type {@code int}.
*/
public static final Class<Integer> TYPE = (Class<Integer>) Class.getPrimitiveClass("int");
/**
* All possible chars for representing a number as a String
*/
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'
};
//十位上的数值，可以根据36，取的十位上数为3
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',
} ;
//个位上的数值，可以根据36，取的个位上数为6
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 int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,99999999, 999999999, Integer.MAX_VALUE };
//避免除法操作，直接比较大小返回位数
static int stringSize(int x) {
for (int i=0; ; i++)
if (x <= sizeTable[i])
return i+1;
}
public Integer(int value) { this.value = value;}
@Native public static final int SIZE = 32;
public static final int BYTES = SIZE / Byte.SIZE;
3）内部私有类
private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer cache[];
static {
<span style="color:#ff0000;">// high value may be configured by property</span>
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
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)
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}
和Short.java和Byte.java一样，都是通过静态代码块初始化缓存对象数组，不过，IntegerCache对象的high可以通过JVM的启动参数设置，缺省为127。
4）初始化方法
构造函数：
public Integer(String s) throws NumberFormatException {
this.value = parseInt(s, 10);
}
public Integer(int value) {
this.value = value;
}
其他方法：
public static int parseInt(String s, int radix); public static int parseInt(String s) throws NumberFormatException; public static Integer valueOf(String s, int radix) throws NumberFormatException; public static Integer valueOf(String s) throws NumberFormatException; public static Integer valueOf(int i); public static int parseUnsignedInt(String s) throws NumberFormatException; public static int parseUnsignedInt(String s, int radix);
观察“其他方法”的代码，难点主要在下面的方法：
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;//设不同进制下的极限值
while (i < len) {
<span style="color:#ff0000;"> // Accumulating negatively avoids surprises near MAX_VALUE
//该方法返回该字符根据进制对应的数字，比如2进制1那么返回1，比如16进制F那么返回15
digit = Character.digit(s.charAt(i++),radix);//i++:0->1
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;</span>
}
} else {
throw NumberFormatException.forInputString(s);
}
return negative ? result : -result;
}
该代码段例子分析：
F1F（16进制）
（1）digit 15；result *= radix -》0；result -= digit -》 -15
（2）digit 1；result *= radix -》-15*16；result -= digit -》 -15*16 - 1(相当于累加)
（3）digit 15；result *= radix -》-(15*16+1)*16；result -= digit -》 -(15*16+1)*16 - 15
（4）-（ -(15*16+1)*16 - 15）= （15*15+1）*16 + 15
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);
}
}
例子：
Integer a11 = 3;
Integer a6 = Integer.parseInt("11",2);
Integer a7 = Integer.parseUnsignedInt("80000000",16);
Integer a9 = Integer.valueOf(3);
Integer a10 = new Integer(3);
boolean b1 = a6==a9;//true
boolean b2 = a6==a11;//true
boolean b3 = a6==a10;//false
初始化对象进行使用parseInt或者valueOf方法，如果值在IntegerCache范围内，可以直接获取对象。
5）一些toXXXString方法：
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];//二进制
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] = '-';
}
//从buf数组的charPos位置开始截取33-charPos字符转换为String类型
return new String(buf, charPos, (33 - charPos));
}
分析该函数之前，我们用十进制转换为二进制来解释下算法：比如10，10%2 商5余0;5%2 商2余1；2%2 商1余0；2%1 商0余1，把余数倒序拼接1010,1010就是二进制。i++号在后面的意思是先赋值然后自身加1；++i在前面的是先自身加1后赋值；--同样。
public static String toUnsignedString(int i, int radix);
public static String toHexString(int i);
public static String toOctalString(int i);
public static String toBinaryString(int i){
return toUnsignedString0(i, 1);
}
//前面4个方法都是调用该方法，shift：次幂
private static String toUnsignedString0(int val, int shift) {
// assert shift > 0 && shift <=5 : "Illegal shift value";
//获取有效的二进制的位数 31位二进制
int mag = Integer.SIZE - Integer.numberOfLeadingZeros(val);
//一个算法，可以根据二进制的有效位数，求出8进制或者16进制的位数
//比如0100 0000 shift=3 -》 （7+（3-1））/3 = 3 八进制需要3位表示
int chars = Math.max(((mag + (shift - 1)) / shift), 1);
char[] buf = new char[chars];
formatUnsignedInt(val, shift, buf, 0, chars);
// Use special constructor which takes over "buf".
return new String(buf, true);
}
//转换为2进制，高位-》低位，直至遇到1停止，0的个数
//比如0000 0000 0000 0000 0000 0000 0000 0001-》31
//比如0100 0000 0000 0000 0000 0000 0000 0001-》1
//比如0010 0000 0000 0000 0000 0000 0000 0001-》2
public static int numberOfLeadingZeros(int i) {
// HD, Figure 5-6
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;
}
static int formatUnsignedInt(int val, int shift, char[] buf, int offset, int len) {
int charPos = len;
int radix = 1 << shift;
int mask = radix - 1;
do {
//val&mask 可以获取末尾使用该进制表示的数值
//例子 假设16进制
// 1010 1010 1010 1000
//& 0000 0000 0000 1111
// 0000 0000 0000 1000
buf[offset + --charPos] = Integer.digits[val & mask];
val >>>= shift;
} while (val != 0 && charPos > 0);
return charPos;
}
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];
// 将Integer数读入到char[]数组
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
// 处理超过2的16次方的大数
while (i >= 65536) {
q = i / 100;
// really: r = i - (q * 100);
// 假设 65536 那么等于 36，根据36取的十位数和个位数上的数值
r = i - ((q << 6) + (q << 5) + (q << 2));
//655
i = q;
//个位 6
buf [--charPos] = DigitOnes[r];
//十位 3
buf [--charPos] = DigitTens[r];
}
// Fall thru to fast mode for smaller numbers
// assert(i <= 65536, i);
// 处理<2的16次方的大数
for (;;) {
q = (i * 52429) >>> (16+3);//i*52429/524288 ≈0.1000003。相当于i/10
//获取个位的数字
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;
}
}
一个例子：
for(int i = 0;i<65536;i++){
int q = (i * 52429) >>> (16+3);//相当于q = i/10;
int j = i/10;
if(q != j){
Out.println(false);
}
}
输出：true
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