Logic of converting strings to integer

Question

Hi all,
I know that , I can convert a string into a number using API , Ineger.parseInt() . But i want to know the reall algoritham behind that
.Can anybody explane that ?
Basically I am interested in
  String s = "123";
  int i = Integer.parseInt(s);
  But i want to know what is the logic running behind this ?. Can anybody please help me. ...?
Thanks in advance....

Answer 1

The decompiled version parseInt() from Integer class
     * Parses the string argument as a signed decimal integer. The
     * characters in the string must all be decimal digits, except that
     * the first character may be an ASCII minus sign <code>'-'</code>
     * (<code>'&#92;u002D'</code>) to indicate a negative value. The resulting
     * integer value is returned, exactly as if the argument and the radix
     * 10 were given as arguments to the
     * {@link #parseInt(java.lang.String, int)} method.
     * @param s        a <code>String</code> containing the <code>int</code>
     *             representation to be parsed
     * @return     the integer value represented by the argument in decimal.
     * @exception  NumberFormatException  if the string does not contain a
     *               parsable integer.
    public static int parseInt(String s) throws NumberFormatException {
     return parseInt(s,10);
     * Parses the string argument as a signed integer in the radix
     * specified by the second argument. The characters in the string
     * must all be digits of the specified radix (as determined by
     * whether {@link java.lang.Character#digit(char, int)} returns a
     * nonnegative value), except that the first character may be an
     * ASCII minus sign <code>'-'</code> (<code>'&#92;u002D'</code>) to
     * indicate a negative value. The resulting integer value is returned.
     * <p>
     * An exception of type <code>NumberFormatException</code> is
     * thrown if any of the following situations occurs:
     * <ul>
     * <li>The first argument is <code>null</code> or is a string of
     * length zero.
     * <li>The radix is either smaller than
     * {@link java.lang.Character#MIN_RADIX} or
     * larger than {@link java.lang.Character#MAX_RADIX}.
     * <li>Any character of the string is not a digit of the specified
     * radix, except that the first character may be a minus sign
     * <code>'-'</code> (<code>'&#92;u002D'</code>) provided that the
     * string is longer than length 1.
     * <li>The value represented by the string is not a value of type
     * <code>int</code>.
     * </ul><p>
     * Examples:
     * <blockquote><pre>
     * parseInt("0", 10) returns 0
     * parseInt("473", 10) returns 473
     * parseInt("-0", 10) returns 0
     * parseInt("-FF", 16) returns -255
     * parseInt("1100110", 2) returns 102
     * parseInt("2147483647", 10) returns 2147483647
     * parseInt("-2147483648", 10) returns -2147483648
     * parseInt("2147483648", 10) throws a NumberFormatException
     * parseInt("99", 8) throws a NumberFormatException
     * parseInt("Kona", 10) throws a NumberFormatException
     * parseInt("Kona", 27) returns 411787
     * </pre></blockquote>
     * @param      s   the <code>String</code> containing the integer
     *                representation to be parsed
     * @param      radix   the radix to be used while parsing <code>s</code>.
     * @return     the integer represented by the string argument in the
     *             specified radix.
     * @exception  NumberFormatException if the <code>String</code>
     *              does not contain a parsable <code>int</code>.
    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;
    }