What's the point of having a default value in sharedPref.getString?

Question

I\'m accessing my Android apps SharedPreferences via

private val sharedPref = PreferenceManager.getDefaultSharedPreferences(context)`


        

Answer 1

Consider this in a such way: Every String preference in SharedPreferences can exist or not and can be null or not. So the code

val lat: String = sharedPref.getString("MyKey", "Default") ?: "Not Set"

will return:

• Default if the preference with this Key doesn't exists (means there is no mapping for this Key)
• Not Set if the preference exists, but is null (mapping Key to null created)
• any other value if the preference exists and the value of the mapping isn't null.

Answer 2

SharedPreferences is an abstraction over Key/Value databasing provided by Google, how you use it is up to you. If you dislike this syntax, then create a wrapper or extension for your getString(). Just to give an example:

fun PreferenceManager.getStringTheFancyWay(key: String, defaultValue: String): String {
    return getString(key, defaultValue) ?: defaultValue
}
val value = getStringTheFancyWay("a", "b")

Personally I dislike this, because null allows for a better control flow over non-existing keys.

This is how I use SharedPreferences

val preferences = PreferenceManager.getDefaultSharedPreferences(context)
val value = preferences.getString("username", null)
if (value != null) {
    toast("Hello $value")
} else {
    toast("Username not found")
}

or

preferences.getString("username", null)?.let { username ->
    toast("Hello $username")
}

Notice the difference?

Answer 3

The fact is simple, just imagine you haven't saved any value regarding to that key(for your case 'MyKey') and tried to get the value for that key(for your case 'MyKey'). What will SharedPreference return ? It will simply return the default value.

You will see that, you must assign null or any other string to default for String type, 0 or any other int value to default for integer type and true or false default value for bolean type. I hope you got the answer.

Answer 4

It's because kotlin Null-Safety is kick in when reading the following code:

val lat: String = sharedPref.getString("MyKey", "Default")

if you visit the SharedPreferences code, you can see the following code:

@Nullable
String getString(String key, @Nullable String defValue);

which is give us a probability to use null as defValue parameter. So, Kotlin try to guard it and give you the matching error:

"Type mismatch. Required String, found String?"

You can fix the problem by enabling nullable for your String variable with:

val lat: String? = sharedPref.getString("MyKey", "Default")

though this against Kotlin type system purpose:

Kotlin's type system is aimed at eliminating the danger of null references from code, also known as the The Billion Dollar Mistake.