ListPreference：使用字符串数组作为条目，使用整数数组作为条目值不起作用

Question

I'm using in a settings.xml file a ListPreference. I want to show the user a list of 3 options to select. When the user chooses one of the options in the Settings, I get this error:

java.lang.NullPointerException
    at android.preference.ListPreference.onDialogClosed(ListPreference.java:264)
    at android.preference.DialogPreference.onDismiss(DialogPreference.java:381)
    at android.app.Dialog$ListenersHandler.handleMessage(Dialog.java:1228)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4424)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
    at dalvik.system.NativeStart.main(Native Method)

This is the code of the ListPreference:

<ListPreference
    android:entries="@array/date_alignement"
    android:entryValues="@array/date_alignement_values"
    android:key="settings_date_alignement"
    android:summary="@string/settings_date_alignement_summary"
    android:title="@string/settings_date_alignement_title" />

And here are the arrays I use to populate the entries:

<string-array name="date_alignement">
    <item>"Top"</item>
    <item>"Center"</item>
    <item>"Bottom"</item>
</string-array>
<integer-array name="date_alignement_values">
    <item >0</item>
    <item >1</item>
    <item >2</item>
</integer-array>

In my onSharedPreferenceChanged I use those values in this way:

@Override
public void onSharedPreferenceChanged(
            SharedPreferences sharedPreferences, String key) {          

        //Text
        mAlignment =  mPrefs.getInt(PREF_ALIGNMENT, 1);
        switch (mAlignment) {
        case 0:
            offsetY = mHeight/3.0f;
            break;

        case 2:
            offsetY = mHeight*2.0f/3.0f;
            break;

        default:
            offsetY = mHeight/2.0f;
            break;
        }
}

If I use another string-array for the entryValues it works. For example if I use the same string array as values and entries:

    android:entries="@array/date_alignement"
    android:entryValues="@array/date_alignement"

then I have to change my code a little but it works:

        if(mAlignment.equalsIgnoreCase("center")) {
            offsetY = mHeight/2.0f;
        } else if(mAlignment.equalsIgnoreCase("top")) {
            offsetY = mHeight/3.0f;
        } else if(mAlignment.equalsIgnoreCase("bottom")) {
            offsetY = mHeight*2.0f/3.0f;
        }

Why I can't use a string-array and an integer-array for a ListPreference entries and values?

Answer 1

The answer is simple: because the Android is designed this way. It just uses String arrays for both entries and entry values and that's all. And I can't see any problem in this fact, since you can easily convert a String to an int using the Integer.parseInt() method. Hope this helps.

Answer 2

The answer is listed in List Preference Documentation .

int findIndexOfValue (String value)

will return the index for given value and the argument is taken as a String, so the entryValues array should be a string array to get this work.

Answer 3

You can convert your entry values to strings to keep ListPreference happy and then convert them to int s when talking to the persistent data store.

1. When setting the entry values, use strings instead of ints: "1", "2", "3" etc
2. Make a custom IntListPreference that persists the values as ints
3. In your preferences.xml file, change the <ListPreference> into a <your.app.package.IntListPreference>

IntListPreference.java

Here's a sample implementation. Tested and working on AndroidX Preference 1.0.0.

public class IntListPreference extends ListPreference {

    public IntListPreference(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    public IntListPreference(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    public IntListPreference(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public IntListPreference(Context context) {
        super(context);
    }

    @Override
    protected boolean persistString(String value) {
        int intValue = Integer.parseInt(value);
        return persistInt(intValue);
    }

    @Override
    protected String getPersistedString(String defaultReturnValue) {
        int intValue;

        if (defaultReturnValue != null) {
            int intDefaultReturnValue = Integer.parseInt(defaultReturnValue);
            intValue = getPersistedInt(intDefaultReturnValue);
        } else {
            // We haven't been given a default return value, but we need to specify one when retrieving the value

            if (getPersistedInt(0) == getPersistedInt(1)) {
                // The default value is being ignored, so we're good to go
                intValue = getPersistedInt(0);
            } else {
                throw new IllegalArgumentException("Cannot get an int without a default return value");
            }
        }

        return Integer.toString(intValue);
    }

}

Answer 4

The following worked for me:

String objectName = prefs.getString("listPrefMelodyYd1", "");
switch (objectName.toUpperCase()) {
    case "1":
        playSound(catSound);
        break;
    case "2":
        playSound(dogSound);
        break;
    case "3":
        playSound(cowSound);
        break;
}