public static void main(String [] strings){

    ValidatorFactory validatorFactory = Validation.buildDefaultValidatorFactory();
    Validator validator = validatorFactory.getValidator();

    Set<ConstraintViolation<User>> constraints = validator.validate(new User("Stas","StasPassword","Stas.Stanis88@gmail.com"));

    for (ConstraintViolation<User> constraint:constraints) {

        System.out.println(constraint.getMessage());

    }

public class User {

public String getFirstName() {
    return firstName;
}

private void setFirstName(String firstName) {
    this.firstName = firstName;
}

public String getLastName() {
    return lastName;
}

private void setLastName(String lastName) {
    this.lastName = lastName;
}

public String getPassword() {
    return password;
}

private void setPassword(String password) {
    this.password = password;
}

public String getEmail() {
    return email;
}

private void setEmail(String email) {
    this.email = email;
}

public String getCustomerType() {
    return customerType;
}

private void setCustomerType(String customerType) {
    this.customerType = customerType;
}

public String getNickName() {
    return nickName;
}

private void setNickName(String nickName) {
    this.nickName = nickName;
}

public User(){}

public User(String nickName,String password, String email){

    setEmail(email);
    setNickName(nickName);
    setPassword(password);
}

public User(String firstName, String lastName, String nickName ,String email, String password,  String customerType) {

    setCustomerType(customerType);
    setEmail(email);
    setFirstName(firstName);
    setLastName(lastName);
    setNickName(nickName);
    setPassword(password);
}

@Override
public String toString() {
    return "User{" +
            "firstName='" + firstName + ''' +
            ", lastName='" + lastName + ''' +
            ", password='" + password + ''' +
            ", email='" + email + ''' +
            ", customerType='" + customerType + ''' +
            ", nickName='" + nickName + ''' +
            '}';
}

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    User user = (User) o;

    if (!getFirstName().equals(user.getFirstName())) return false;
    if (!getLastName().equals(user.getLastName())) return false;
    if (!getPassword().equals(user.getPassword())) return false;
    if (!getEmail().equals(user.getEmail())) return false;
    if (!getCustomerType().equals(user.getCustomerType())) return false;
    return getNickName().equals(user.getNickName());

}

@Override
public int hashCode() {
107:    int result = getFirstName().hashCode(); // Проблемма возникает здесь!
    result = 31 * result + getLastName().hashCode();
    result = 31 * result + getPassword().hashCode();
    result = 31 * result + getEmail().hashCode();
    result = 31 * result + getCustomerType().hashCode();
    result = 31 * result + getNickName().hashCode();
    return result;
}

@NotEmpty(message = "This field can't be empty")
@NotBlank(message = "This field can't be empty")
@Length(min = 0,max = 20,message = "The length of this field can be only 0-20")
private String firstName;

@NotEmpty(message = "This field can't be empty")
@NotBlank(message = "This field can't be empty")
@Length(min = 0,max = 20,message = "The length of this field can be only 0-20")
private String lastName;

@NotEmpty(message = "This field can't be empty")
@NotBlank(message = "This field can't be empty")
@Length(min = 8,max = 16,message = "Wrong password length: length of this field can be only 8-16")
private String password;

@NotEmpty(message = "This field can't be empty")
@NotBlank(message = "This field can't be empty")
@Email(message = "Wrong format for email field!" , regexp = "[A-Za-z.0-9]+@[A-Za-z.]+\.[a-z]{2,4}")
private String email;

@NotEmpty(message = "This field can't be empty")
@NotBlank(message = "This field can't be empty")
private String customerType;

@NotEmpty(message = "This field can't be empty")
@NotBlank(message = "This field can't be empty")
@Length(min = 4,max = 10,message = "Wrong nickName: The length of this field can be only between 4-10")
private String nickName;

 }