The correct way of implementing getItemId() in RecyclerView.Adapter

Question

I have generic class

public abstract class BaseAdapter<T> extends RecyclerView.Adapter {
  private List<T> itemsList = new ArrayList<>();
  //other override methods

  @Override
    public long getItemId(int position) {
        return position;
    }
}

What is the correct way to implementing getItemId()? I think that return position like in many example is not correct.

score 15 · Accepted Answer · edited Oct 15 '17 at 23:21

15

Create a base interface that has a method that returns a type long eg.
```
interface BaseInterface{
    long getId(); 
}
```

Change

abstract class BaseAdapter<T> extends RecyclerView.Adapter

to

abstract class BaseAdapter<T extends BaseInterface> extends RecyclerView.Adapter {

Note: The signature changed to T extends BaseInterface

Replace

@Override
public long getItemId(int position) {
    return position;
}

with

@Override
public long getItemId(int position) {
    return itemsList.get(position).getId();
}

edited Oct 15 '17 at 23:21

blizzard

5,275
2
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48

answered Mar 07 '17 at 11:23

Bubunyo Nyavor

2,511
24
36

itemsList has type . And has only default lists methods – Sky Mar 07 '17 at 11:58
and itemsList.get(position). has objects methods – Sky Mar 07 '17 at 11:59
The Google I/O app itself returns position as fallback, I wonder if this is OK https://github.com/google/iosched/blob/e4364eec0d8f358215aee30cce2b4836ec169b5a/lib/src/main/java/com/google/samples/apps/iosched/myio/MyIOAdapter.java#L93 – David May 24 '18 at 11:20
What about `hashCode()` instead of `getId()`? – Big McLargeHuge May 31 '19 at 19:09
`hashCode()` implementation cannot be enforced. If an interface forgets to implement it, it will not throw an error. – Bubunyo Nyavor Jun 01 '19 at 07:50

score 4 · Answer 2 · answered Mar 07 '17 at 11:36

4

In the List you could only return the Id of the Item available at specific row as mentioned by Google documents:

getItemId

Get the row id associated with the specified position in the list.

But that's not the case with RecyclerView, in RecyclerView you have to ensure that you either return a unique Id for each Item or In case of no Stable Id you should return RecyclerView.NO_ID (-1). Please Refrain from returning values that are not stable. (An Stable value would be a unique value that does not change even if position of dataset changes)

answered Mar 07 '17 at 11:36

Keivan Esbati

3,376
1
22
36

1

Exactly, the behavior of [getItemId](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.Adapter.html#getItemId(int)) in RecyclerView.Adapter differs from [getItemId](https://developer.android.com/reference/android/widget/ArrayAdapter.html#getItemId(int)) in ArrayAdapter. – Hasan El-Hefnawy Apr 07 '19 at 13:14

Amir Hossein · Answer 3 · 2020-10-26T21:22:20.443

1

you should implement this methode: as you see you must tell the Adapter that you implemented that by setHasStableIds(true);

class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {
MyAdapter() {
    setHasStableIds(true); // **very importent**
}

@Override
public long getItemId(int position) {
    // requires unique value, it means need to keep the same value
    // even if the item position has been changed.
    return mItems.get(position).getId();
}

}

edited Oct 26 '20 at 21:22

answered May 26 '20 at 13:17

Amir Hossein

392
3
11

can you explain why this code gives a static value? to me it looks like `mItems.get(position)` returns an instance of your model class , which has a a function returning some integer id. why would your model return the same id again and again? If you are using dbs or network, then usually the `id` is unique and `getId()` returns a unique value everytime – ansh sachdeva Oct 22 '20 at 06:24
You're right. By "static" I mean unique value for each item. Not a same value. – Amir Hossein Oct 23 '20 at 10:09
Then please change the word `static` to `unique` , as this gives a totally different meaning, and newbies like me spend 10 minutes on this answer to get even more confused :P – ansh sachdeva Oct 25 '20 at 20:47

The correct way of implementing getItemId() in RecyclerView.Adapter

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