class Solution {
    public boolean isNumber(String s) {
        ValidateNumber validateNumber = new NumberValidators();
        return validateNumber.validate(s);
    }

    public interface ValidateNumber {
        public boolean validate(String s);
    }

    abstract class ValidateNumberTemplate implements ValidateNumber {
        public boolean validate(String s) {
            if(checkIsEmptyString(s)) {
                return false;
            }

            s = checkAndProcessHeader(s);

            return doValidate(s);
        }

        private checkAndProcessHeader(String s){
            s = s.trim();
            if (s.startsWith("+") || s.startsWith("-")) {
                return s.substring(1);
            }
            return s;
        }

        public abstract boolean doValidate(String s);
    }

    public class NumberValidators implements ValidateNumber {
        private static List<ValidateNumber> validators = new ArrayList<ValidateNumber>();

        public NumberValidators {
            addValidators();
        }

        public boolean validateNumber(String s) {
            for (ValidateNumber validator : validators) {
                if (!validator.validateNumber(s)) {
                    return false;
                }
            }
            return true;
        }

        private void addValidators() {
            ValidateNumber validator = new IntegerValidator();
            validators.add(validator);
        }
    }

    public class IntegerValidator extends ValidateNumberTemplate {
        public boolean doValidate(String s) {
            for (int i = 0; i < s.length(); i++) {
                if(!Character.isDigit(s.charAt(i))) {
                    return false;
                }
            }
        }
    }
}