Saving custom array in shared preferences

Question

I'm about to write a small app where I have pretty much all the data stored in a custom array which contains of list views. I don't wanna save the data in a different form. This array I like to save in shared preferences. I head a look for other answered questions and tried different ways but none of then worked for me.

In the main activity, I have two small methods:

public class MainActivity extends AppCompatActivity {

public static ArrayList<Item> items = new ArrayList<Item>();
SharedPreferences mPrefs = getBaseContext().getSharedPreferences("KEY12", Context.MODE_PRIVATE);
Gson gson = new Gson();

[...]

protected void savePrefs() {

    SharedPreferences.Editor editor = mPrefs.edit();
    String json = gson.toJson(items);
    System.out.println(json);
    editor.putString("items_prefs", json);
    editor.commit();

}

protected void loadPrefs() {

    String empty_list = gson.toJson(new ArrayList<Item>());

    items = gson.fromJson(mPrefs.getString("KEY12", empty_list),
            new TypeToken<ArrayList<Item>>() {}.getType());


}
}

I'm calling the loadPrefs(); method in onCreate and savePrefs(); in onResume so everytime when I come back to the main activity, the preferences should be updated.

The error that I get is:

FATAL EXCEPTION: main Process: com.example.android.calc123, PID: 30579 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.calc123/com.example.android.calc123.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6077) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) at com.and roid.internal.os.ZygoteInit.main(ZygoteInit.java:755) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)' on a null object reference at com.example.android.calc123.MainActivity.(MainActivity.java:61) at java.lang.Class.newInstance(Native Method) at android.app.Instrumentation.newActivity(Instrumentation.java:1078) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2538) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6077) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCa ller.run(ZygoteInit.java:865) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

How can I fix this problem or - asking in a different way - what am I doing wrong when dealing with shared preferences?

Answer 1

As the error log say you call getSharedPreferences in a null pointer (getBaseContext)

put in OnCreate mPrefs = getBaseContext().getSharedPreferences("KEY12", Context.MODE_PRIVATE);