Why this null pointer error is comming?

Question

I was making a list of favorite activity in the android which uses arraylist as a storing place,but when i was triggering the event by the button press,null pointer error is coming. Why is it so...??? Here is the code of my Kinematics activity where the button is being pressed.

public class Kinematics extends AppCompatActivity {
private DrawerLayout drawerLayout;
private NavigationView navigationView;
private ActionBarDrawerToggle phy_law_toggel;
private Toolbar toolbar;
private WebView webView;
private ScrollView scrollview;
private FloatingActionButton fab;
private ImageButton fav;

@Override
protected void onCreate(Bundle savedInstanceState) {
    final Favouritepage favr = new Favouritepage();
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_kinematics);
    Toolbar toolbar = (Toolbar) findViewById(R.id.phy_lawtoolbar);
    setSupportActionBar(toolbar);
    setTitle(R.string.Kinematics);
    drawerLayout = (DrawerLayout) findViewById(R.id.phy_draw);
    navigationView = (NavigationView) findViewById(R.id.nav_view_phy);

    phy_law_toggel = new ActionBarDrawerToggle(this,drawerLayout,R.string.open, R.string.Close);
    drawerLayout.addDrawerListener(phy_law_toggel);
    toolbar = (Toolbar) findViewById(R.id.phy_lawtoolbar);
    setSupportActionBar(toolbar);
    phy_law_toggel.syncState();
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    navigationView.setItemIconTintList(null);
    webView = (WebView) findViewById(R.id.phy_law_web);
    fav = (ImageButton) findViewById(R.id.fav);
    webView.getSettings().setJavaScriptEnabled(true);
    webView.getSettings().setBuiltInZoomControls(true);
    webView.getSettings().setSupportZoom(true);
    final String kine = this.getClass().getSimpleName();
    webView.getSettings().setDisplayZoomControls(true);
    webView.loadUrl("file:///android_asset/mathscribe/Kinematics.html");
    fab = (FloatingActionButton) findViewById(R.id.fab);
    scrollview = (ScrollView) findViewById(R.id.scroll);

    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            webView.scrollTo(0, 0);
        }
    });
    fav.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            favr.mfavlist.add(kine);
        }
    });


}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    if (phy_law_toggel.onOptionsItemSelected(item)) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}}

Here, i am making an object of my FavoritePage class and trying to store the element in the arrayList of FavoritePage.

Here is my FavoritePage class

public class Favouritepage extends AppCompatActivity {

public ArrayList<String> mfavlist;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_favouritepage);
    mfavlist = new ArrayList<String>();

}}

And here is the XML of kinematics,

<android.support.v4.widget.DrawerLayout                                
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/phy_draw"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:context="com.example.alpit.formula2.Kinematics">


<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v7.widget.Toolbar
        android:id="@+id/phy_lawtoolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:popupTheme="@style/AppTheme.NoActionBar.PopupOverlay">


    </android.support.v7.widget.Toolbar>


    <WebView
        android:id="@+id/phy_law_web"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignParentEnd="true"
        android:layout_alignParentStart="true"
        android:layout_below="@+id/phy_lawtoolbar"></WebView>

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="56dp"
        android:layout_height="56dp"
        android:layout_alignBottom="@+id/phy_law_web"
        android:layout_alignParentBottom="true"
        android:layout_alignParentEnd="true"
        android:layout_gravity="bottom|end"
        android:layout_marginBottom="43dp"
        android:layout_marginEnd="30dp"
        android:src="@drawable/fabss"
        app:backgroundTint="#F57F17">  

</android.support.design.widget.FloatingActionButton>

    <ImageButton
        android:id="@+id/fav"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:layout_above="@+id/phy_law_web"
        android:layout_alignParentEnd="true"
        android:background="@color/colorPrimary"
        android:src="@drawable/fav" />


</RelativeLayout>


<android.support.design.widget.NavigationView
    android:id="@+id/nav_view_phy"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:background="#FFAB40"
    app:menu="@menu/navigation_menu"
    app:theme="@style/NavigationTheme" />

crashlog

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.alpit.formula2, PID: 22744
              java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference
                  at com.example.alpit.formula2.Kinematics$2.onClick(Kinematics.java:64)
                  at android.view.View.performClick(View.java:4802)
                  at android.view.View$PerformClick.run(View.java:20102)
                  at android.os.Handler.handleCallback(Handler.java:810)
                  at android.os.Handler.dispatchMessage(Handler.java:99)
                  at android.os.Looper.loop(Looper.java:189)
                  at android.app.ActivityThread.main(ActivityThread.java:5529)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:372)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:950)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745)

@Henry , I dont think so, i might be lacking some concept though. Do review my code. — Alpit Anand, Feb 22 '17 at 19:35
@AlpitAnand Yes, your variables are null, so please debug your own code to see "how to fix it". Many many questions about null variables, yet, no one takes the time to understand how to fix it themselves — OneCricketeer, Feb 22 '17 at 19:40
@cricket_007,its been two days, still trying to figure out,I might need another set of eyes to look at my code,still trying though.. — Alpit Anand, Feb 22 '17 at 19:43

score 1 · Answer 1 · answered Feb 22 '17 at 19:27

1

Add this line after setContentView in your Kinematic.

ArrayList<String> mfavlist = new ArrayList<String>();

edit this line of your code

fav.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
       mfavlist.add(kine);
    }
});

answered Feb 22 '17 at 19:27

Narendra Sorathiya

3,770
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Thanks for the reply though,but we are adding the name of activities in `FavoriteList`, I want my ArrayList in `FavoriteList`,instead of having arraylist locally in every activity. – Alpit Anand Feb 22 '17 at 19:31
You can not access your arraylist from One Activity to other Activity – Narendra Sorathiya Feb 22 '17 at 19:32
there are 2 ways : pass your arraylist to Kinematic Activity with Intent OR create Application class & replace an arraylist at there. – Narendra Sorathiya Feb 22 '17 at 19:33
When your Favouritepageactivity finished then your arraylist will be null so its fired NullPointerException – Narendra Sorathiya Feb 22 '17 at 19:35
Okie, thanks i will give a try, but not even by creating a object i can access it..?? – Alpit Anand Feb 22 '17 at 19:39
you need to create Application class & initialize your arraylist at there & then after you can access that Arraylist from Anywhere in your application – Narendra Sorathiya Feb 22 '17 at 19:42
1

https://www.intertech.com/Blog/androids-application-class/ this can help you to create an application class – Narendra Sorathiya Feb 22 '17 at 19:43
Yaah sure, i always do, just let me check if it works – Alpit Anand Feb 22 '17 at 19:46
working my solution ? – Narendra Sorathiya Feb 22 '17 at 20:12
I will soon as return from collage... – Alpit Anand Feb 23 '17 at 09:02

score 0 · Answer 2 · answered Feb 22 '17 at 19:43

Here is my FavoritePage class

public class Favouritepage extends AppCompatActivity {

... Meanwhile in your other class...

final Favouritepage favr = new Favouritepage();

Never ever new an Activity class (because onCreate is never called, therefore the list is null)

Basically, I do not think FavouritePage should extend Activity at all.

Learn more about POJO classes

public class Favouritepage {

    public ArrayList<String> mfavlist = new ArrayList<String>();

    public Favouritepage() { 
        // Empty constructor 
    }
}

Or, if it needs does need to be an Activity class, you will have to find a different way to "share" that list between activities.

Shouldn't be. Like I said, `onCreate` is never called. In your code, `mfavlist = new ArrayList();` is never ran. — OneCricketeer, Feb 22 '17 at 20:53

Why this null pointer error is comming?

2 Answers2