How to correctly load saved SharedPreferences from another activity on startup?

Question

I thought the error I'm catching is for only the first time running (where there's still no SharedPreferences saved, but the error pops up everytime I run the app (in MainActivity):

Attempt to invoke virtual method 'int com.android.test.helper.PreferencesHelper.load(java.lang.String)' on a null object reference

The same function however works and loads the value under that key (key_foo) successfully in the OptionsActivity where that option is saved. MainActivity runs first when opening the app immediately being replaced by OptionsActivity after some more setup.

MainActivity.java:

public class MainActivity {
    private PreferencesHelper preferencesHelper;

    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        try {
            preferencesHelper.load("key_foo");
            recreate();
        }
        catch(Exception e) {
            Log.d("ERROR", e.getMessage());
        }
        startOptionsActivity();
    }
}

PreferencesHelper.java:

public class PreferencesHelper {
    private Activity context;
    private SharedPreferences sharedPref;
    private SharedPreferences.Editor editor;

    public PreferencesHelper (Activity activity){
        context = activity;
        sharedPref = context.getSharedPreferences("file", 0);
    }
    public int load(String key){
        sharedPref = context.getSharedPreferences("file", 0);
        return sharedPref.getInt(key, -1);
    }
    public void save(String key, int value){
        editor = sharedPref.edit();
        editor = putInt(key, value);
        editor.apply();
    }
}

OptionsActivity.java:

public class OptionsActivity {
    private PreferencesHelper preferencesHelper;

    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_options);
        
        preferencesHelper.save("key_foo", 42);
        recreate();

        try {
            preferencesHelper.load("key_foo");
        }
        catch(Exception e) {
            Log.d("ERROR", e.getMessage());
        }
    }
}

score 2 · Accepted Answer · answered Mar 15 '21 at 04:23

Attempt to invoke virtual method 'int com.android.test.helper.PreferencesHelper.load(java.lang.String)' on a null object reference

NullPointerException is thrown when program attempts to use an object reference that has the null value. You must declare PreferencesHelper in onCreate() section.

private PreferencesHelper preferencesHelper;

@Override
protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    preferencesHelper = new PreferencesHelper(this);

How to correctly load saved SharedPreferences from another activity on startup?

1 Answers1