implementation 'com.google.android.material:material:1.4.0'

<com.google.android.material.textfield.TextInputLayout
    style="@style/Widget.MaterialComponents.TextInputLayout.OutlinedBox.ExposedDropdownMenu"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <AutoCompleteTextView
        android:id="@+id/dropdown_menu"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="none"
        android:hint="Select an item" />

</com.google.android.material.textfield.TextInputLayout>

AutoCompleteTextView dropdownMenu = findViewById(R.id.dropdown_menu);
String[] items = {"Item 1", "Item 2", "Item 3"};
ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_dropdown_item_1line, items);
dropdownMenu.setAdapter(adapter);

<style name="AppTheme" parent="Theme.MaterialComponents.Light">
    <item name="textInputStyle">@style/Widget.MaterialComponents.TextInputLayout.OutlinedBox.ExposedDropdownMenu</item>
</style>