public final class Integer extends Number implements Comparable
从类定义可以看出Integer不可以被继承，继承了Number类，实现了Comparable
@SuppressWarnings("unchecked")
public static final Class TYPE = (Class) Class.getPrimitiveClass("int");
Integer.TYPE取得是基本数据类型int的class
public static String toString(int i, int radix)
这个方法是返回对应进制的转为string的数
public static String toUnsignedString(int i, int radix) {
return Long.toUnsignedString(toUnsignedLong(i), radix);
}
要理解这两个方法要知道怎么从十进制转到其他进制 ，以及二进制，四进制，八进制和十六进制如何互相转换
public static String toHexString(int i) {
return toUnsignedString0(i, 4);
}
public static int numberOfLeadingZeros(long i)
获取数字中有多少个0
/**
* 将integer转换为无符号数字 shift 二进制1 八进制3 十六进制4
*/
private static String toUnsignedString0(int val, int shift) {
// assert shift > 0 && shift <=5 : "Illegal shift value";
//32减去val中为0的部分的长度
int mag = Integer.SIZE - Integer.numberOfLeadingZeros(val);
//如果是二进制的话 那就是mag
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);
}
static int formatUnsignedInt(int val, int shift, char[] buf, int offset, int len) {
int charPos = len;
//表示进制
int radix = 1 << shift;
//进制-1 二进制1 八进制111 十六进制1111
int mask = radix - 1;
//将数字转化为对应的进制
do {
buf[offset + --charPos] = Integer.digits[val & mask];
val >>>= shift;
} while (val != 0 && charPos > 0);
return charPos;
}
将string类型转为int
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");
}
//如果radix小于2 抛出异常
if (radix < Character.MIN_RADIX) {
throw new NumberFormatException("radix " + radix +
" less than Character.MIN_RADIX");
}
//如果radix大于36 抛出异常
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();
//MAX_VALUE = 0x7fffffff;
int limit = -Integer.MAX_VALUE;
int multmin;
int digit;
if (len > 0) {
char firstChar = s.charAt(0);
if (firstChar < '0') { // 有可能以+或者-开头
if (firstChar == '-') {//如果以-开头，那么是负数 ，除此之外，如果不以+开头，异常
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;//猜测multmin是数字的长度
while (i < len) {//将字符转为数字
// 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;
}
Integer类中的缓存
表示缓存从-128到127的数字
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
int h = 127;
//获取系统配置文件中的high配置
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() {}
}
自动拆装箱用的就是valueOf()
如果在缓存命中范围内 那么就用缓存的，不然new一个
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
hashcode返回本身
public static int hashCode(int value) {
return value;
}
equals方法
public boolean equals(Object obj) {
if (obj instanceof Integer) {
return value == ((Integer)obj).intValue();
}
return false;
}