基于jdk1.8的Java源码详解Integer

public final class Integer extends Number implements Comparable

public Integer(int value) {
this.value = value;
}

public Integer(String s) throws NumberFormatException {
this.value = parseInt(s, 10);
}

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.MAX_RADIX) {
throw new NumberFormatException("radix " + radix +
" greater than Character.MAX_RADIX");
}
int result = 0;
// //是否为负数
boolean negative = false;
int i = 0, len = s.length();
//这里加个负号是防止数据溢出，int的数值范围 -2的31次方到2的31次方减一
int limit = -Integer.MAX_VALUE;
//最小基数
int multmin;
//十进制数字
int digit;
if (len > 0) {
char firstChar = s.charAt(0);
//第一个字符小于0，有可能是"-","+"或其他字符
if (firstChar <'0') { // Possible leading "+" or "-"
//为负数
if (firstChar == '-') {
negative = true;
limit = Integer.MIN_VALUE;
} else if (firstChar != '+')//非数字
throw NumberFormatException.forInputString(s);
if (len == 1) // Cannot have lone "+" or "-"
throw NumberFormatException.forInputString(s);
i++;
}
/**
* 最小基数，主要防止 result *= radix; 这个操作时数据过大
* 导致数据丢失的问题，因为所以带符号32位int类型整数为-2147483648~2147483647
*/
multmin = limit / radix;
while (i 

public static void main(String[] args) {
Integer i1 = 100;
Integer i2 = 100;
Integer i3 = 200;
Integer i4 = 200;
Integer i5 = Integer.valueOf(100);
Integer i6 = Integer.valueOf(100);
Integer i7 = new Integer(100);
System.out.println("i1 == i2 的结果是：" + (i1 == i2));
System.out.println("i3 == i4 的结果是：" + (i3 == i4));
System.out.println("i5 == i6 的结果是：" + (i5 == i6));
System.out.println("i1 == i5 的结果是：" + (i1 == i5));
System.out.println("i1 == i7 的结果是：" + (i1 == i7));
}

i1 == i2 的结果是：true
i3 == i4 的结果是：false
i5 == i6 的结果是：true
i1 == i5 的结果是：true
i1 == i7 的结果是：false

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

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
//缓存范围最小（也是默认范围）为 （-128）~ 127，如果配置java.lang.Integer.IntegerCache.high
//high 的值可从配置文件中读取
int h = 127;
String integerCacheHighPropValue =
sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
if (integerCacheHighPropValue != null) {
try {
int i = parseInt(integerCacheHighPropValue);
//获取配置文件和127之间的最大值
i = Math.max(i, 127);
// Maximum array size is Integer.MAX_VALUE
//最大值范围
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;
//创建缓存数组
cache = new Integer[(high - low) + 1];
int j = low;
//将数字缓存起来默认 -128 ~ 127
for(int k = 0; k = 127;
}
private IntegerCache() {}
}

/**
* The value of the {@code Integer}.
*
* @serial
*/
private final int value;
public boolean equals(Object obj) {
if (obj instanceof Integer) {
return value == ((Integer)obj).intValue();
}
return false;
}
public int intValue() {
return value;
}

public static void main(String[] args) {
Integer i1 = 200;
Integer i2 = 200;
System.out.println("i1 == i2 的结果是：" + i1.equals(i2)); //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
//每次循环获取后两位数
while (i >= 65536) {
q = i / 100;
// really: r = i - (q * 100);
//使用位移运算的效率高于乘法运算，r为后两位数
r = i - ((q <<6) + (q <<5) + (q <<2));
i = q;
//获取后两位数的个位
buf [--charPos] = DigitOnes[r];
//十位
buf [--charPos] = DigitTens[r];
}

// Fall thru to fast mode for smaller numbers
// assert(i <= 65536, i);
//每次只取个位数
for (;;) {
//相当于i*(52429/524288)=i*0.10000038146972656=i*0.1=i/10
//这里选 52429 和 2的19次方相除，得到的结果精度更加高，更加接近于 i/10的结果
//之所以要这样转换，是因为在计算机运算中位移的效率 > 乘法效率 > 除法效率
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;
}
}