A string headded by "+" causes NumberFormatException

Exception occurs with such string on :
   Integer.valueOf
   Long.ValueOf
   Short.ValueOf

No problem:
   Double.valueOf
   Float.valueOf
 
 
Sample program:

public class test2{
   public static void main(String aa[]){
      Double doubleValue;
      Float floatValue;
      Integer integerValue;
      Long longValue;
      Short shortValue;
      doubleValue = Double.valueOf("+3.21"); // OK
      floatValue = Float.valueOf("+3.21"); // OK
      integerValue = Integer.valueOf("+321"); // NumberFormatException
      longValue = Long.valueOf("+321"); // NumberFormatException
      shortValue = Short.valueOf("+321"); // NumberFormatException
   }
}