java/79266.html">java;">public class IntegerDemo {  public static void main(String[] args) {    String num = null;    System.out.println( Integer.parseInt(num));// Exception java.lang.NumberFormatException    System.out.println( Integer.valueOf(num));// Exception java.lang.NumberFormatException    System.out.println( String.valueOf(num)); //輸出null    num = "";    System.out.println( Integer.parseInt(num)); // Exception java.lang.NumberFormatException    System.out.println( Integer.valueOf(num)); // Exception java.lang.NumberFormatException    System.out.println( String.valueOf(num));//空串，啥也不輸出  }}

 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);        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;  }