验证字符串是否只有数字

Question

My Java application 'A' is getting mobile number as String from another java application. So after app A gets the mobile number string, I need to validate if that mobile number String has only numbers in it. For validating that I have used a simple logic as below,

public static boolean containsOnlyDigits(String str, int n) {
    for (int i = 1; i < n; i++) {
        if (!Character.isDigit(str.charAt(i))) {
            return false;
        }
    }
    return true;
}

I am checking from i=1 as 1st character will be '+' for country code. This approach is O(n). There is another approach where we can use Double.parseDouble(str) . This will throw NumberFormatException so that we can catch and consider it is an Alphanumeric String.

Which of these approaches are best based on performance?

Answer 1

You could try, remove the + if not useful:

 /**
 * Assuming that a phone number is of an international format, having `+` sign as prefix
 * and with no spaces in between the numbers, validates if a string is a valid phone number.
 * 
 * @param phone
 * @return resulting boolean
     */
private boolean isValidPhoneNumber(String phone) {
    if(phone == null) return false;
    
    return Pattern.matches("\\+\\d{11,15}", phone);
}

Answer 2

I am checking from i=1 as 1st character will be '+' for country code.

You can use the regex, \\+\\d{n - 1} which means the first character is + and the remaining n - 1 characters are digits. It also means that the total length of the string should be n .

public class Main {
    public static void main(String[] args) {
        // Tests
        System.out.println(containsOnlyDigits("+123456789", 10));
        System.out.println(containsOnlyDigits("+123456789", 9));
        System.out.println(containsOnlyDigits("+123456789", 8));
        System.out.println(containsOnlyDigits("-123456789", 9));
        System.out.println(containsOnlyDigits("123456789", 9));
        System.out.println(containsOnlyDigits("ABC123456", 9));
    }

    public static boolean containsOnlyDigits(String str, int n) {
        String regex = "\\+\\d{" + (n - 1) + "}";
        return str.matches(regex);
    }
}

Output:

true
false
false
false
false
false

Answer 3

Using regular expressions is costly. For this reason, you can easily solve this problem using the lambda notation in Java 8.

boolean numericControl = str.chars().allMatch(x -> Character.isDigit(x));