array of sub Sets from parent Set ?

[kri shan]

name, age and qualification are the columns in the database. I accessed whole data from the database and put in the Set(java.util.Set). Now i want to form array of Sets based on qualification value(qualification is the key for forming array of Sets).

[Ankit Garg]

kri shan wrote:name, age and qualification are the columns in the database. I accessed whole data from the database and put in the Set(java.util.Set).

But how can you insert three values into a set?? Do you have a POJO class like User which you insert into the set?? I also didn't get the actual question that you asked...

[amitabh mehra]

How are you inserting data into the set? Is it a tuple.. I mean as some class objects?

[kri shan]

Yes, POJO class like User which inserts data into the set

[Rob Spoor]

There are two different types of sub sets.

The first is the snapshot (pseudo code):

// Set<MyType> allObjects is the base
Set<MyType> subSet = new HashSet<MyType>(); // or TreeSet, or LinkedHashSet; basically the same type as allObjects
for (MyType type: allObjects)
{
    if (type matches some criteria)
    {
        subSet.add(type);
    }
}
return subSet;

The second is much harder, and is a view on the original set. If the original set changes, the view changes as well. This is what SortedSet.subSet, SortedSet.headSet and SortedSet.tailSet are required to return. You can check TreeSet (and effectively TreeMap) of how this can be implemented.
AbstractList.subList also does something similar.

[kri shan]

Rob,

based on your first soultion:
I want to create array of subSets based on Qualification values. Because i do not know how many different qulification type values are in the database.

[Rob Spoor]

I would go for a Map<Qualification,Set<MyType>>, combined with a method that returns a Set<MyType> given a Set<MyType> and Qualification. (This method can be private, or an instance method on Qualification, or whatever you choose.)

You can store the entire set with a null key, or of course as a separate field. For all qualifications, you can retrieve them from the map as follows:

// the Map is called subSets, the entire Set is called allTypes
public Set<MyType> getSubSet(Qualification qualification)
{
    // if you store the Set with a null key, check if qualification is null first; otherwise you may overwrite your base Set!
    Set<MyType> subSet = subSets.get(qualification);
    if (subSet == null)
    {
        subSet = getSubSet(allTypes, qualification);
        subSets.put(qualification, subSet);
    }
    return subSet;
}

This is a lazy initialization implementation; if you never need a sub set for a qualification, you simply never initialize it. If you do, you only initialize it once and cache it afterwards.

Note that I made an abstraction of your qualification; perhaps in your code it will be a simple String, or an int / long (in which case you use Integer / Long as key type). The important thing is, that this code should work regardless of your representation choice.

[kri shan]

Hi Rob, sbSet = getSubSet(allTypes, qualification); here getSubSet() is taking two arguments allTypes and qualification. Is it separate function ?

[Rob Spoor]

Yes it is. It is the method that actually does the filtering (e.g. like my previous post). But like I said, it could be a method of class Qualification, in which case the call would be qualification.getSubSet(allTypes).