Passing reference of bean into another bean

Passing reference of bean into another bean

I have a class A which contain the following
class A
{
private HashSet<Long> at = new HashSet<Long>();
and it has a constructor like this
A()
{
//set is being initialsised here
this.at.add(new Long(44));
this.at.add(new Long(34));
this.at.add(new Long(54));
this.at.add(new Long(55));
}
Now following is the spring xml bean defined for it ...
<bean id="aa" class="com.abc.A">
<property name="readPermissionGroup">
<set>
<value>03</value>
<value>13</value>
<value>54</value>
<value>55</value>
</set>
</property>
</bean>
Now please advise how can I add the above bean aa into bb as the bean bb
contain the complete defination of class A