@Entity(name = "accounts")
public class Account {
    @Id
    @GeneratedValue(
            strategy = GenerationType.SEQUENCE,
            generator = "sequence-generator"
    )
    @SequenceGenerator(
            name = "sequence-generator",
            sequenceName = "acc_rub_seq",
            allocationSize = 1
    )
    private int accID;

    private String accNum;

    public Account() {
        this.accNum = "xxxxx" + String.valueOf(accID);
    }
...
}