How to find out if all the characters in a string are digits?

Question

Hi, I want to write a program that is boolean & it returns true if all the charactrs in a string are digit. i have written a prgoram which returns true if String has digits where as I want it to return true if whole string is digit. can anyone help please?
public class stringD {
          //construct String
private String str;          
public boolean isDigit (String str) {
boolean answer=true;
for( int i = 0; i < str.length(); i++ )     {
     if( str.charAt( i ) < (int)'A' )     {
          answer=true;
     else {
          answer= false;
     }//if & else
}//end for
return (answer);
}//end boolean
public static void main (String [] args)     {
          stringD str = new stringD ();
          System.out.println("String is Thank you 1234");
          System.out.println("If all the characters in string are digits. It will be true: " + str.isDigit("Thank you 1234"));
} //end main

Answer 1

boolean b = true;
for (int i = 0; b && i < str.length(); i++) {
  if (str.charAt(i) > '9' || str.charAt(i) < '0') {
    b = false;
if (b) {
  System.out.println(str+" is a number");
} else {
  System.out.println(str+" is not a number");
}