Strings.java

package modern.challenge;

public final class Strings {

    private Strings() {
        throw new AssertionError("Cannot be instantiated");
    }

    // Note: For Unicode supplementary characters use codePointAt() instead of charAt()
    //       and codePoints() instead of chars()
    
    public static boolean containsOnlyDigitsV1(String str) {

        if (str == null || str.isBlank()) {
            // or throw IllegalArgumentException
            return false;
        }
        
        for (int i = 0; i < str.length(); i++) {
            if (!Character.isDigit(str.charAt(i))) {
                return false;
            }
        }
        
        return true;
    }

    public static boolean containsOnlyDigitsV2(String str) {

        if (str == null || str.isBlank()) {
            // or throw IllegalArgumentException
            return false;
        }

        return str.matches("[0-9]+");
    }

    public static boolean containsOnlyDigitsV3(String str) {

        if (str == null || str.isBlank()) {
            // or throw IllegalArgumentException
            return false;
        }

        return !str.chars()
                .anyMatch(n -> !Character.isDigit(n));
    }
}