public class Activity extends BaseEntity {
[...]

    @NotNull
    @Column(columnDefinition = "a_type", name = "type")
    @Enumerated(STRING)
    @Type(type = "pg-enum", parameters = { @Parameter(name = "enumClassName", value = "......schema.AType") })
    private Set<AType> type = new HashSet<AType>;
}

CREATE TABLE activity
(
    ....
    type a_type[],
    ...
);