App crashing when trying to end session.. null object reference

Question

I am trying to implement a logout button to the application which I am developing. When I click logout from the navigation menu session.logoutUser(); is called, and causing the following error in my IDE;

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.ravi.myapplication, PID: 32694
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.ravi.myapplication.SessionHandler.logoutUser()' on a null object reference
    at com.example.ravi.myapplication.BaseActivity$1.onNavigationItemSelected(BaseActivity.java:57)
    at android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:154)
    at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822)
    at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:171)
    at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:973)
    at android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:342)
    at android.view.View.performClick(View.java:6597)
    at android.view.View.performClickInternal(View.java:6574)
    at android.view.View.access$3100(View.java:778)
    at android.view.View$PerformClick.run(View.java:25885)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

I am getting a warning for private field never assigned when I call the session class at the start of the Base Activity class. If anyone could shed some light on why this is happening it would be thoroughly appriciated! The code for the classes are below;

BaseActivity.java

package com.example.ravi.myapplication;

import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.design.widget.NavigationView;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.MenuItem;
import android.widget.Spinner;

public class BaseActivity extends AppCompatActivity {
private SessionHandler session;
DrawerLayout drawerLayout;
ActionBarDrawerToggle actionBarDrawerToggle;
Toolbar toolbar;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_base);
    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);

    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawerLayout, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawerLayout.setDrawerListener(actionBarDrawerToggle);



    navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(MenuItem item) {
            final String appPackageName = getPackageName();

            switch (item.getItemId()) {

                case R.id.nav_dashboard:
                    Intent dash = new Intent(getApplicationContext(), MainActivity.class);
                    startActivity(dash);
//                        finish();
                    drawerLayout.closeDrawers();
                    break;


                case R.id.nav_about_us:
                    Intent anIntent = new Intent(getApplicationContext(), AboutUS.class);
                    startActivity(anIntent);
//                        finish();
                    drawerLayout.closeDrawers();
                    break;

                case R.id.nav_logout:
                    session.logoutUser();
                    Intent i = new Intent(getApplicationContext(), LoginActivity.class);
                    startActivity(i);
                    break;


            }
            return false;
        }
    });

}

@Override
protected void onPostCreate(Bundle savedInstanceState) {
    super.onPostCreate(savedInstanceState);

    actionBarDrawerToggle.syncState();
}

@Override
public void finish() {
    super.finish();
    overridePendingTransitionExit();
}

@Override
public void startActivity(Intent intent) {
    super.startActivity(intent);
    overridePendingTransitionEnter();
}

/**
 * Overrides the pending Activity transition by performing the "Enter" animation.
 */
protected void overridePendingTransitionEnter() {
    overridePendingTransition(R.anim.slide_from_right, R.anim.slide_to_left);
}

/**
 * Overrides the pending Activity transition by performing the "Exit" animation.
 */
protected void overridePendingTransitionExit() {
    overridePendingTransition(R.anim.slide_from_left, R.anim.slide_to_right);
}
}

SessionHandler.java

package com.example.ravi.myapplication;

import android.content.Context;
import android.content.SharedPreferences;

import java.util.Date;

public class SessionHandler {
private static final String PREF_NAME = "UserSession";
private static final String KEY_USERNAME = "username";
private static final String KEY_EXPIRES = "expires";
private static final String KEY_FULL_NAME = "full_name";
private static final String KEY_EMPTY = "";
private Context mContext;
private SharedPreferences.Editor mEditor;
private SharedPreferences mPreferences;

public SessionHandler(Context mContext) {
    this.mContext = mContext;
    mPreferences = mContext.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
    this.mEditor = mPreferences.edit();
}

/**
 * Logs in the user by saving user details and setting session
 */
public void loginUser(String username, String fullName) {
    mEditor.putString(KEY_USERNAME, username);
    mEditor.putString(KEY_FULL_NAME, fullName);
    Date date = new Date();

    //Set user session for next 7 days
    long millis = date.getTime() + (7 * 24 * 60 * 60 * 1000);
    mEditor.putLong(KEY_EXPIRES, millis);
    mEditor.commit();
}

/**
 * Checks whether user is logged in
 */
public boolean isLoggedIn() {
    Date currentDate = new Date();

    long millis = mPreferences.getLong(KEY_EXPIRES, 0);

    /* If shared preferences does not have a value
     then user is not logged in
     */
    if (millis == 0) {
        return false;
    }
    Date expiryDate = new Date(millis);

    /* Check if session is expired by comparing
    current date and Session expiry date
    */
    return currentDate.before(expiryDate);
}

/**
 * Fetches and returns user details
 */
public User getUserDetails() {
    //Check if user is logged in first
    if (!isLoggedIn()) {
        return null;
    }
    User user = new User();
    user.setUsername(mPreferences.getString(KEY_USERNAME, KEY_EMPTY));
    user.setFullName(mPreferences.getString(KEY_FULL_NAME, KEY_EMPTY));
    user.setSessionExpiryDate(new Date(mPreferences.getLong(KEY_EXPIRES, 0)));

    return user;
}

/**
 * Logs out user by clearing the session
 */
public void logoutUser(){
    mEditor.clear();
    mEditor.commit();
}

}

score 1 · Accepted Answer · answered Jan 04 '19 at 21:10

1

to session field in class BaseActivity any value is not assigned, so it is null

answered Jan 04 '19 at 21:10

Bartosz Wardziński

6,185
1
19
30

Do you know how i'd go about assigning the correct value to it? I'm confused as I used it in the same way in what an application which was similar and it worked! – bbowesbo Jan 04 '19 at 21:15
You have to create instance of `SessionHandler` class and assign it to field `sessions` – Bartosz Wardziński Jan 04 '19 at 21:17
Thanks, it is now working! – bbowesbo Jan 04 '19 at 21:25

App crashing when trying to end session.. null object reference

1 Answers1