Realm.io Android best approach to get last 20 items from a table

问题

In a table of let say 100 items, which is the best approach to get the last 20 objects.

One way I can think of is to load all the objects , reverse the array , create a new array and loop from the results for 20 times filling the new array and return it.

Something like as follows :

 public ArrayList<DataObject> getLastItems (int qty){

    RealmResults<DataObject>results = realm.where(DataObject.class).findAll();

    Collections.reverse(results);

    ArrayList<DataObject>arrayList = new ArrayList<>();

    for (int i = 0; i == qty; i++){

        arrayList.add(results.get(i));

    }

    return arrayList;
}

Is there a better faster way to do this in android using realm.io ?

Update

this is so far how this is handled..

  public ArrayList<DataObject> getLastItems (int qty){

    RealmResults<DataObject>results = realm.where(DataObject.class).findAll();
    ArrayList<DataObject> arrayList = new ArrayList<>();
    for (int i = results.size(); i > Math.max(results.size() - 20, 0) ; i--) {
        arrayList.add(results.get(i-1));
    }

    return arrayList;
}

回答1:

Also note that Realm tables are unordered. Think of them as a bag where you put your data. This means that if you want the last 20 items you inserted you will need to add a field to contain the insertion time. Doing this will also allow you to achieve the result you want very efficiently:

RealmResults<DataObject>results =
    realm.where(DataObject.class)
         .findAllSorted("timestamp", RealmResults.SORT_ORDER_DESCENDING);

for (int i = 0; i < 20; i++) {
    // do magic here
}

回答2:

This question has been almost closed, but I found another way to figure out that situation, I don't know if is the best option or a good practice, @ChristianMelchior can say better than me about that (I saw your collaboration with realm project)... well, codes say more than words.

RealmResults<Appointment> appointments; 
List<Appointment> appointments = appointments.subList(0, appointments.size());

As we can see, answering your question Johan, you can just change the index to get all objects that you want!

回答3:

RealmResults only create objects you actually use, so iterating over all of them to reverse the list would be really bad performance wise. Instead as bmunk described, you should just find the proper index and start from there, like the below. Note that Realms are unordered so without a sort, the items returned would not be well defined.

public ArrayList<DataObject> getLastItems (int qty){
    RealmResults<DataObject>results = realm.where(DataObject.class).findAll().sort("fieldName");
    ArrayList<DataObject> arrayList = new ArrayList<>();
    for (int i = Math.max(result.size() - 20, 0); i < results.size() ; i++) {
        arrayList.add(results.get(i));
    }

    return arrayList;
}

回答4:

Do like this

Realm mRealm=Realm.getDefaultInstance();

RealmResults<Example> list= mRealm.where(Example.class).findAll();
list.subList(list.size()-20,list.size());

回答5:

You should try to avoid copying objects unless strictly needed. Once you have done a findAll() you already got an ordered list. You could then just use max(results.size()-20, 0) to know which index to start your iteration from instead of copying.

来源：https://stackoverflow.com/questions/28229866/realm-io-android-best-approach-to-get-last-20-items-from-a-table

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