Validate if string has only numbers

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?

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);
}

> 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

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));