How does Integer.parseInt(string) actually work?

Question

Was asked this question recently and did not know the answer. From a high level can someone explain how Java takes a character / String and convert it into an int.

M

Answer 1

The source code of the Java API is freely available. Here's the parseInt() method. It's rather long because it has to handle a lot of exceptional and corner cases.

public static int parseInt(String s, int radix)
    throws NumberFormatException
{
    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, max = s.length();
int limit;
int multmin;
int digit;

if (max > 0) {
    if (s.charAt(0) == '-') {
    negative = true;
    limit = Integer.MIN_VALUE;
    i++;
    } else {
    limit = -Integer.MAX_VALUE;
    }
    multmin = limit / radix;
    if (i < max) {
    digit = Character.digit(s.charAt(i++),radix);
    if (digit < 0) {
        throw NumberFormatException.forInputString(s);
    } else {
        result = -digit;
    }
    }
    while (i < max) {
    // 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);
}
if (negative) {
    if (i > 1) {
    return result;
    } else {    /* Only got "-" */
    throw NumberFormatException.forInputString(s);
    }
} else {
    return -result;
}
}