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Only digits

Write a program to find out if the string contains only digits.

The long method would be to scan the string and determine whether each character is a digit. 

The short but not so good method would be to convert this string to Integer object. If the string has non-digits, the method would throw an exception.

The third method is using regular expression. Java String class has a method called matches() which checks for regular expressions. 

String s1 = "123";
if (s1.matches("\\d+"){
  /*****/
}

Here \\d meta character indicates digit. + indicates one or more. So we are saying if the string is composed of 1 or more digits - but nothing else, return true.

Here is the complete program to find if the given string contains only digits.


import java.util.Scanner;

public class OnlyDigits {
    public static void main(String args[]) {
        System.out.println("Enter a string:");
        Scanner scanner = new Scanner(System.in);
        String str = scanner.nextLine();
        boolean onlyDigits =  str.matches("\\d+");
        if(onlyDigits==true)
            System.out.println("The string contains only digits");
        else
            System.out.println("no");
    }
}
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