import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.AutoCompleteTextView;

public class MainActivity extends AppCompatActivity {

    private static final String[] capitales_europeennes = new String[]{
            "Tirana", "Berlin", "Andorre-la-Vieille", "Vienne", "Bruxelles",
            "Minsk", "Sarajevo", "Sofia", "Nicosie", "Zagreb", "Copenhague",
            "Madrid", "Tallinn", "Helsinki", "Paris", "Athènes", "Budapest",
            "Dublin", "Reykjavik", "Rome", "Pristina", "Riga", "Vaduz",
            "Vilnius", "Luxembourg", "Skopje", "La Valette", "Chisinau",
            "Monaco", "Podgorica", "Oslo", "Amsterdam", "Varsovie",
            "Lisbonne", "Bucarest", "Londres", "Moscou", "Belgrade",
            "Bratislava", "Ljubljana", "Berne", "Stockholm", "Prague",
            "Kiev", "Vatican"
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_dropdown_item_1line, capitales_europeennes);

        AutoCompleteTextView autoCompleteTextView = (AutoCompleteTextView) findViewById(R.id.capitales);
        autoCompleteTextView.setAdapter(arrayAdapter);

        System.out.println("Threshold : " + autoCompleteTextView.getThreshold());
        autoCompleteTextView.setThreshold(1);
    }
}

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.formation.autocomplete.MainActivity">

    <AutoCompleteTextView
        android:id="@+id/capitales"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="Capitale européenne"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>