Android: PreferenceActivity: Why is this 'simple' logic check on a stored preference value not evaluating?

Question

Following an example from Professional Android 2 Application Development I set up a pref class as follows:

//Preferences.java
public class Preferences extends PreferenceActivity{
    public static final String PREF_SPEED_UNIT ="PREF_SPEED_UNIT";
    @Override
    public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        addPreferencesFromResource(R.xml.userpreferences);
    }  
}

In my activity class:

// main.java 

private static final int SHOW_PREFERENCES = 1;

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == SHOW_PREFERENCES)
        updateFromPreferences();
        if (resultCode == Activity.RESULT_OK) {
        }   
}

private void updateFromPreferences() {
    Log.d(TAG, "updateFromPreferences()");
    Context context = getApplicationContext();
    SharedPreferences prefs = PreferenceManager
            .getDefaultSharedPreferences(context);
    // Set the SpeedMode string using object reference to custom view
    mSpeedMode = prefs.getString(Preferences.PREF_SPEED_UNIT, "MPH");
    speedview.SpeedMode = mSpeedMode;  
}
// Using have a Button view that pops the options for testing
private OnClickListener optionbuttonCL = new OnClickListener() {
    public void onClick(View v) {
        Context context = getApplicationContext();
        Intent i = new Intent(context, Preferences.class);
        startActivityForResult(i, SHOW_PREFERENCES);
    }
};

The issue is here:

    // SpeedView.java
    if (SpeedMode == "MPH"){
        // set speed in miles/hour
    }
    if (SpeedMode == "KPH"){
        //set speed in kilometers/hour
    }

It works, however if I switch speed modes to KPH (default is MPH), the display no longer updates.

The bewildering thing is even if SpeedMode is shown to be "MPH" or "KPH" using the Eclipse debugger, the if statements will not execute.

Here is the rest of the code:

//userpreferences.xml
<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"&gt;
<ListPreference android:key="PREF_SPEED_UNIT"
    android:title="Speed Unit" 
    android:summary="Choose between MPH or KPH"
    android:entries="@array/speed_options" 
    android:entryValues="@array/speed_values"
    android:dialogTitle="Speed Unitz" 
    android:defaultValue="MPH" />
</PreferenceScreen>


// arrays.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
<string-array name="speed_options">
    <item>MPH</item>
    <item>KPH</item>
</string-array>
<string-array name="speed_values">
    <item>MPH</item>
    <item>KPH</item>
</string-array>

Thanks

Answer 1

What is speedview.SpeedMode? Where is it declared. I'll bet your problem is

SpeedMode == "MPH"

Change it to

Speedmode.equals("MPH");

Also, by convention in java and languages that use camelCase, variables should be lowercase.

By variables being lowercase, I mean the first letter of the variable. So if you had a class MyClass. You would declare it

MyClass myClass;

That way you know you're not trying to access class methods instead of instance methods.

This is a good explanation on why you need to use .equals() instead of ==

http://leepoint.net/notes-java/data/expressions/22compareobjects.html