Checking if a String Contains only Digits in Java

In many programming scenarios, user input plays a crucial role. When dealing with numerical data, it becomes essential to ensure that the input contains only digits. Whether you’re validating a user’s age, a phone number, or any other numeric identifier, being able to check if a string contains only digits is a valuable skill. This prevents unexpected errors and enhances the reliability of your programs.

Java’s Character Class

Java provides a handy class called Character that offers methods for character-related operations. One of these methods, isDigit(char ch), checks if a character is a digit. We can leverage this method to build a simple and efficient solution for our task.

public class DigitCheckerExample {

    public static boolean containsOnlyDigits(String str) {

        for (char ch : input.toCharArray()) {

            if (!Character.isDigit(ch)) {
                return false;
            }

        }

        return true;
    }

    public static void main(String[] args) {

        String numericString = "12345";
        String mixedString = "abc123";

        System.out.println("Is '" + numericString + "' a numeric string? " + containsOnlyDigits(numericString));
        System.out.println("Is '" + mixedString + "' a numeric string? " + containsOnlyDigits(mixedString));
    }
    
}

In this example, the containsOnlyDigits method takes a string as input, converts it to a character array, and iterates through each character. If any character is not a digit, the method returns false. Otherwise, if all characters are digits, it returns true.

Regular Expressions for Digit Validation

Another approach to check if a string contains only digits involves using regular expressions. Java’s String class provides a matches method that can be used with regular expressions to validate the entire string against a specific pattern.

Here’s how you can achieve digit validation using regular expressions:

public class DigitCheckerRegexExample {

    public static boolean containsOnlyDigits(String input) {
        return input.matches("\\d+");
    }

    public static void main(String[] args) {

        String numericString = "12345";
        String mixedString = "abc123";

        System.out.println("Is '" + numericString + "' a numeric string? " + containsOnlyDigits(numericString));
        System.out.println("Is '" + mixedString + "' a numeric string? " + containsOnlyDigits(mixedString));
    }
}

In this example, the regular expression “\d+” checks if the string contains one or more digits. The matches method returns true if the entire string matches this pattern.

Conclusion

Ensuring that a string contains only digits is a common requirement in Java programming. By using the Character class or regular expressions, you can implement efficient and reliable solutions. Whether you prefer the simplicity of character iteration or the conciseness of regular expressions, Java provides the tools to meet your digit validation needs. For more content, please subscribe to our newsletter.