autocomplete email

The autocomplete by default works when you enter the first character and it matches the first character of one or more elements defined in Array .

In this case you need the autocomplete to be activated when you enter the character @ , for this you need a Adapter custom in which the filter that determines the suggestions to be displayed in the view is modified.

String[] email = {"@gmail.com", "@hotmail.com", "@yahoo.com"};
//Adapter<String> adapter = new ArrayAdapter<String> (this, android.R.layout.select_dialog_item,email);

CustomFilterAdapter adapter = new CustomFilterAdapter(this, android.R.layout.simple_list_item_1, /*Convert Array to List*/ new ArrayList<>(Arrays.asList(email)));

AutoCompleteTextView actv = (AutoCompleteTextView) findViewById(R.id.auto);
actv.setThreshold(1);
actv.setAdapter(adapter);

I suggest this Adapter :

import android.content.Context;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.Filter;
import android.widget.TextView;
import java.util.ArrayList;

public class CustomFilterAdapter extends ArrayAdapter<String> {

    private static final String TAG = "CustomFilterAdapter";
    private ArrayList<String> items;
    private ArrayList<String> itemsAll;
    private ArrayList<String> suggestions;
    private int viewResourceId;

    public CustomFilterAdapter(Context context, int viewResourceId, ArrayList<String> items) {
        super(context, viewResourceId, items);
        this.items = items;
        this.itemsAll = (ArrayList<String>) items.clone();
        this.suggestions = new ArrayList<String>();
        this.viewResourceId = viewResourceId;
    }

    public View getView(int position, View convertView, ViewGroup parent) {
        View v = convertView;
        if (v == null) {
            LayoutInflater vi = (LayoutInflater) getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            v = vi.inflate(viewResourceId, null);
        }
        String customer = items.get(position);
        if (customer != null) {
            TextView customerNameLabel = (TextView)v;
            if (customerNameLabel != null) {
                customerNameLabel.setText(customer);
            }
        }
        return v;
    }

    @Override
    public Filter getFilter() {
        return nameFilter;
    }

    Filter nameFilter = new Filter() {
        public String convertResultToString(Object resultValue) {
            String str = (String)resultValue;
            return str;
        }

        @Override
        protected FilterResults performFiltering(CharSequence constraint) {
            if (constraint != null){
                String word = constraint.toString();
                String beforeCiotola;
                //find character @
                if(word != null && word.indexOf("@") != -1) {
                    String newWord = word.substring(word.indexOf("@"));
                    try{
                        beforeCiotola = word.substring(0, word.indexOf("@"));
                    }catch (Exception ex) {
                        Log.e(TAG, ex.getMessage());
                        beforeCiotola ="";
                    }
                    suggestions.clear();
                    for (String customer : itemsAll) {
                        if(customer.toLowerCase().startsWith(newWord.toString().toLowerCase())){
                            suggestions.add(beforeCiotola+customer);
                        }
                    }
                    FilterResults filterResults = new FilterResults();
                    filterResults.values = suggestions;
                    filterResults.count = suggestions.size();
                    return filterResults;
                } else {
                    return new FilterResults();
                }
            }else {
                return new FilterResults();
            }
        }

        @Override
        protected void publishResults(CharSequence constraint, FilterResults results) {
            ArrayList<String> filteredList = (ArrayList<String>) results.values;
            if(results != null && results.count > 0) {
                clear();
                for (String c : filteredList) {
                    add(c);
                }
                //Update adapter.
                notifyDataSetChanged();
            }
        }
    };

}

To get something similar to: