Java - check if string only contains numbers

In this article, we would like to show you how to check if the string only contains numbers in Java.

1. Using regex

In this example, we use a regular expression (regex) with Pattern.matcher() to check if the strings contain only numbers.

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Example {

    public static void main(String[] args) {
        String numbers = "1234";
        String text = "ABC";

        String regex = "^[0-9]+$";  // regex to check if string contains only digits
        Pattern pattern = Pattern.compile(regex);  // compiles the regex

        // find match between given string and pattern
        Matcher matcherNumbers = pattern.matcher(numbers);
        Matcher matcherText = pattern.matcher(text);

        // return true if the string matched the regex
        Boolean numbersMatches = matcherNumbers.matches();
        Boolean textMatches = matcherText.matches();

        System.out.println(numbersMatches); // true
        System.out.println(textMatches);    // false
    }
}

Output:

true
false

2. Using Character.isDigit()

In this example, we create a function that loops through the string and checks if each character is a digit with Character.isDigit(char ch) method.

Practical example:

public class Example {

    public static void main(String[] args) {
        String numbers = "1234";
        String text = "ABC";

        System.out.println(onlyNumbers(numbers));     // true
        System.out.println(onlyNumbers(text));        // false
    }

    public static boolean onlyNumbers(String string) {
        if (string == null || string.isEmpty()) {
            return false;
        }
        for (int i = 0; i < string.length(); ++i) {
            if (!Character.isDigit(string.charAt(i))) {
                return false;
            }
        }
        return true;
    }
}

Output:

true
false