//1. 
public static boolean isNumeric(String string){
    Pattern pattern = Pattern.compile("[0-9]*");
    return pattern.matcher(string).matches();   
}



//2.

   public static boolean isNumeric(String str){
        for (int i = str.length();--i>=0;){  
           if (!Character.isDigit(str.charAt(i))){
              return false;
             }     
        }
       return true;
  }

     
//3.
    public static boolean isNumeric(String str){
        for(int i=str.length();--i>=0;){
        int chr=str.charAt(i);
        if(chr<48 || chr>57)
            return false;
        }
       return true;
    }