Validate method not working

[Gabriel Malimpensa]

Hi.

I've created a validate method for an attribute but when I insert the
entity that contains such attribute, the validate is not called by the
interface. I don't know what is wrong. Someone can help?

Code:
    public String validatePrice(Double price) {
        if (price >= 0) {
            return null;
        } else {
            return "The price must not be negative!";
        }
    }

Thanks.

Gabriel Malimpensa.
São Carlos, São Paulo, Brazil.