Why am I Getting a NullPointerException After Accessing an Object From SharedPreferences?

Question

I saved an object (Account) in SharedPreferences using gson, but after accessing the object in a new Activity, the instance of the object (newAccount) causes my use of newAccount.returnName() to give a NullPointerException. The error is: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.cashtrack.kennethlee.cashtrack.Account.returnName()' on a null object reference. I understand what a NullPointer error is, but I don't see why I might be getting it since I'm new to Android Studio. The code for the first activity is:

package com.cashtrack.kennethlee.cashtrack;

import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.support.v4.view.MenuItemCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.EditText;
import android.widget.TextView;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.ArrayList;

public class CreateAccount extends AppCompatActivity {

    SharedPreferences accountsPref;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_create_account);
        setTitle("Create New Account");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        accountsPref = getPreferences(MODE_PRIVATE);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_create_account, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            //back button
            case android.R.id.home:
                // app icon in action bar clicked; go home
                Intent intent = new Intent(this, MainScreen.class);
                intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                startActivity(intent);
                return true;

            //save button
            case R.id.action_menu_save:

                // get EditText by id
                EditText inputTxt1 = (EditText) findViewById(R.id.AccountName);
                // Store EditText in Variable
                String name = inputTxt1.getText().toString();

                EditText inputTxt2 = (EditText) findViewById(R.id.StartingBalance);
                double balance = Double.parseDouble(inputTxt2.getText().toString());

                Account newAccount = new Account(name, balance);

                SharedPreferences.Editor editor = accountsPref.edit();
                Gson gson = new Gson();
                String json = gson.toJson(newAccount);
                editor.putString("newAccount", json);
                editor.commit();

                Intent intent2 = new Intent(this, MainScreen.class);
                intent2.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                startActivity(intent2);
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }
}

The code for the activity that is accessing the object and causing the error is:

package com.cashtrack.kennethlee.cashtrack;

import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

import com.google.gson.Gson;

import java.io.FileInputStream;
import java.io.StringWriter;
import java.util.ArrayList;

public class MainScreen extends AppCompatActivity {
    //array list of accounts
    ArrayList<Account> accounts = new ArrayList<>();

    SharedPreferences accountsPref;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_screen);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(MainScreen.this, CreateAccount.class));
            }
        });
        accountsPref = getPreferences(MODE_PRIVATE);

        //  PRINT TO A TEXT BOX
        //setContentView(R.layout.activity_main_screen);
        //TextView textView = (TextView) findViewById(R.id.textView4);
        //textView.setText(hello.returnName());
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main_screen, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onResume() {
        super.onResume();

        Gson gson = new Gson();
        String json = accountsPref.getString("newAccount", "");
        Account newAccount = gson.fromJson(json, Account.class);
        accounts.add(newAccount);


        setContentView(R.layout.activity_main_screen);
        TextView textView1 = (TextView) findViewById(R.id.textView5);
        textView1.setText(accounts.get(0).returnName());

        TextView textView2 = (TextView) findViewById(R.id.textView6);
        textView2.setText(Double.toString(accounts.get(0).returnAvailableBalance()));
    }
}

score 0 · Answer 1 · answered Feb 19 '17 at 09:24

0

I think you must initialize your Account class in onCreate().

Without initialization it is causing null pointer exception.

In Account class you have created constructor with Context parameter?

answered Feb 19 '17 at 09:24

maulik

140
9

I initialized within the case because I get the name and balance variables for the object in there, so I have to initialize after I get the two variables. I also don't have a context parameter in the account class constructor. What is the purpose of that? – Kenny Lee Feb 19 '17 at 09:34

tahsinRupam · Accepted Answer · 2017-02-19T20:11:28.480

0

You're using MODE_PRIVATE to specify the file name for storing the value. The recommended way is to use by the default mode, without specifying the file name. Like this:

Put value:

accountPref = PreferenceManager.getDefaultSharedPreferences(this);
Gson gson = new Gson();
String json = gson.toJson(Account);
accountPref.edit().putString("newAccount", json).apply();

Get Value:

SharedPreferences accountsPref = PreferenceManager.getDefaultSharedPreferences(this);
if (accountsPref.contains("newAccount")) {
    Gson gson = new Gson();
    String json = accountsPref.getString("newAccount", "");
    Account newAccount = gson.fromJson(json, Account.class);
    accounts.add(newAccount);
}

Hope you like this. :)

edited Feb 19 '17 at 20:11

answered Feb 19 '17 at 09:43

tahsinRupam

5,820
1
15
33

After getting the string json (third line in get value), how do I get the object itself? – Kenny Lee Feb 19 '17 at 18:09
Which object ? Aren't you getting the String value you have put in CreateAccount activity ? Do you want to put new value or remove value from that particular SharedPreference ? – tahsinRupam Feb 19 '17 at 18:18
I want to be able to use the instance of the class Account called newAccount from CreateAccount. I have an arraylist of Account in MainScreen, and I want to add new account from CreateAccount into that arraylist. – Kenny Lee Feb 19 '17 at 18:50
Please checkout the edited answer. – tahsinRupam Feb 19 '17 at 20:12
Thanks a lot!! Got it working now. – Kenny Lee Feb 19 '17 at 22:25
its my pleasure :) – tahsinRupam Feb 19 '17 at 23:26

score 0 · Answer 3 · answered Feb 19 '17 at 09:50

I think that your are having this issues because

getPreferences(int mode)

return a SharedPreferences object for the current Activity, so the String "newAccount" saved in the CreateAccount activity is not retrieved in the MainScreen activity.

So when you use Account newAccount = gson.fromJson(json, Account.class); newAccount is null because json is a empty String.

You may consider use getSharedPreferences(String sharedPreferenceName, int mode) instead of getPreferences(int mode), using the same sharedPreferenceName when you call the method in your activities.

Sorry for my english. I hope it's help.

Why am I Getting a NullPointerException After Accessing an Object From SharedPreferences?

3 Answers3