Java中的源碼
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);//將字符解析爲指定基數的數字
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;
}
例 Integer.parseInt(“101”, 2)
2乘以result 等於0，result加上digit , result等於1
2乘以result 等於2，result加上digit , result等於2
2乘以result 等於4，result加上digit , result等於5
我以前用的都是 1*2^2+0*2^1+1*1^0=5
這個不用進行指數運算，相對來說更加效率
