Beware of Preference Default Values in XML

Various Preferences available in Android make the maintenance of settings straightforward. Even custom Preferences, e.g., for colors, can be created without much effort. By defining PreferenceScreens via XML, default values for each Preference can be set. However, parsing and type conversions of these default values can lead to unexpected and undesired results.

Android Preferences

Using a PreferenceActivity/PreferenceFragment and a PreferenceScreen, setup via XML, Preferences can be easily maintained and presented to the user.

Example

The following code snippet shows an example PreferenceScreen with an EditTextPreference (e.g., saved as res/xml/prefs.xml):

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">

    <EditTextPreference
        android:key="pref_key_edit_text"
        android:title="EditText"/>

</PreferenceScreen>

The EditTextPreference has the key pref_key_edit_text and the title EditText.

In a PreferenceActivity, this PreferenceScreen can be loaded using the addPreferencesFromResource method:

public class SettingsActivity extends PreferenceActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        addPreferencesFromResource(R.xml.prefs);
    }
}

The stored Preference values can be accessed via SharedPreferences.

Preference Types and Underlying Types

Different Preferences use different types for their values. For example, EditTextPreference uses String, CheckBoxPreference boolean, and MultiSelectListPreference a set of Strings.

Default Values

A default value for a Preference can be set using the android:defaultValue attribute:

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">

    <EditTextPreference
        android:key="pref_key_edit_text"
        android:title="EditText"
        android:defaultValue="Default value"/>

</PreferenceScreen>

Alternatively, a reference to a string can be used.

The Problem

Although EditTextPreference stores its value as a String, a default value set in the XML code isn’t simply handled as a String. Instead, during build, default values are parsed, e.g., as floats. This can have unexpected and, usually, undesired effects. Example:

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">

    <EditTextPreference
        android:key="pref_key_edit_text"
        android:title="EditText"
        android:defaultValue="inf"/>

</PreferenceScreen>

The default value inf is parsed as a float, converted to a String, and then stored as the Preference default value. This results in a String value of Infinity being stored instead of inf.

Default values which can’t be parsed as a number are not affected.

Examples

Values Parsed as Float

Values Parsed as Int

Bug Reports

There exist multiple bug reports about this issue.

Solution

Simple: Use a reference to a string for the default value:

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">

    <EditTextPreference
        android:key="pref_key_edit_text"
        android:title="EditText"
        android:defaultValue="@string/my_default_string"/>

</PreferenceScreen>

The official guide also uses references for String values in the examples. However, it does not mention what might happen when default values (like inf) are set directly, without using a String reference.