hibernate Java静态类

Question

在阅读hibernate文档时，我多次看到静态类的使用

下面是documentation中代码的一个节选：

@Entity(name = "SystemUser")
public static class SystemUser {

    @EmbeddedId
    private PK pk;

    private String name;

    //Getters and setters are omitted for brevity
}

@Embeddable
public static class PK implements Serializable {

    private String subsystem;

    private String username;

    public PK(String subsystem, String username) {
        this.subsystem = subsystem;
        this.username = username;
    }

    private PK() {
    }

    @Override
    public boolean equals(Object o) {
        if ( this == o ) {
            return true;
        }
        if ( o == null || getClass() != o.getClass() ) {
            return false;
        }
        PK pk = (PK) o;
        return Objects.equals( subsystem, pk.subsystem ) &&
                Objects.equals( username, pk.username );
    }

    @Override
    public int hashCode() {
        return Objects.hash( subsystem, username );
    }
}

我认为顶级静态类在Java中是不允许的。将此类指示为静态的原因是什么