Why my app crashes with NullPointerException

Question

I've tried to create simple app with using CardView. It is my first steps with android and java so for you this problem might be very easy to fix. Ok then, when i tried to open app on AVD it's crash.

Code:

public class MainActivity extends AppCompatActivity {

GridLayout mainGrid;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mainGrid = (GridLayout) findViewById(R.id.mainGrid);

    setSingleEvent(mainGrid);
}
private void setSingleEvent(GridLayout mainGrid) {
    for (int i = 0; i < mainGrid.getChildCount(); i++) {
        CardView cardView = (CardView) mainGrid.getChildAt(i);
        final int finalI = i;
        cardView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent intent = new Intent(MainActivity.this,bmistart.class);
                intent.putExtra("info","This is activity from card item index  "+finalI);
                startActivity(intent);

            }
        });
    }
}

and the problem:

2020-03-24 21:47:47.986 10136-10136/com.example.mobilefittrener E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.mobilefittrener, PID: 10136
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mobilefittrener/com.example.mobilefittrener.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.widget.GridLayout.getChildCount()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7356)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.widget.GridLayout.getChildCount()' on a null object reference
    at com.example.mobilefittrener.MainActivity.setSingleEvent(MainActivity.java:26)
    at com.example.mobilefittrener.MainActivity.onCreate(MainActivity.java:23)
    at android.app.Activity.performCreate(Activity.java:7802)
    at android.app.Activity.performCreate(Activity.java:7791)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
    at android.os.Handler.dispatchMessage(Handler.java:107) 
    at android.os.Looper.loop(Looper.java:214) 
    at android.app.ActivityThread.main(ActivityThread.java:7356) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

2020-03-24 21:47:48.013 10136-10136/com.example.mobilefittrener I/Process: Sending signal. PID: 10136 SIG: 9

My MainActivity xml:

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    android:weightSum="10"
    tools:context=".MainActivity">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="2">

        <TextView
            android:id="@+id/textGrid"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textColor="@android:color/black"
            android:text="@string/nazwa"
            android:layout_centerInParent="true"
            android:textStyle="bold"
            android:textSize="23dp"
            />
    </RelativeLayout>

    <GridLayout
        android:columnCount="2"
        android:rowCount="3"
        android:alignmentMode="alignMargins"
        android:columnOrderPreserved="false"
        android:layout_weight="8"
        android:padding="14dp"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        >


        <!--Kolumna 1 Wiersz 1-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/bmi"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:clickable="true"
                    android:focusable="true" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 2 Wiersz 1-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:layout_width="wrap_content"
                    android:layout_height="match_parent"
                    android:layout_gravity="center_horizontal"
                    android:src="@drawable/weightlifter" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 1 Wiersz 2-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/climb"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 2 Wiersz 2-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/alarm"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 1 Wiersz 3-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/diet"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 2 Wiersz 3-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/quality"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>



    </GridLayout>
</LinearLayout>

It's my first post, don't be mean please :)

Make sure that: findViewById(R.id.mainGrid) actually exists. — Prajwal Waingankar, Mar 24 '20 at 21:11
It says `mainGrid` does not exist inside the layout `R.layout.activity_main` — EpicPandaForce, Mar 24 '20 at 21:29

score 0 · Answer 1 · answered Mar 24 '20 at 21:26

My MainActivity xml:

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    android:weightSum="10"
    tools:context=".MainActivity">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="2">

        <TextView
            android:id="@+id/textGrid"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textColor="@android:color/black"
            android:text="@string/nazwa"
            android:layout_centerInParent="true"
            android:textStyle="bold"
            android:textSize="23dp"
            />
    </RelativeLayout>

    <GridLayout
        android:columnCount="2"
        android:rowCount="3"
        android:alignmentMode="alignMargins"
        android:columnOrderPreserved="false"
        android:layout_weight="8"
        android:padding="14dp"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        >


        <!--Kolumna 1 Wiersz 1-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/bmi"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:clickable="true"
                    android:focusable="true" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 2 Wiersz 1-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:layout_width="wrap_content"
                    android:layout_height="match_parent"
                    android:layout_gravity="center_horizontal"
                    android:src="@drawable/weightlifter" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 1 Wiersz 2-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/climb"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 2 Wiersz 2-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/alarm"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 1 Wiersz 3-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/diet"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>

        <!--Kolumna 2 Wiersz 3-->
        <androidx.cardview.widget.CardView
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_columnWeight="1"
            android:layout_rowWeight="1"
            android:layout_marginBottom="16dp"
            android:layout_marginLeft="16dp"
            android:layout_marginRight="16dp"
            app:cardElevation="8dp"
            app:cardCornerRadius="8dp">


            <LinearLayout
                android:layout_gravity="center_horizontal|center_vertical"
                android:layout_margin="16dp"
                android:orientation="vertical"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" >

                <ImageView
                    android:src="@drawable/quality"
                    android:layout_gravity="center_horizontal"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />


            </LinearLayout>

        </androidx.cardview.widget.CardView>



    </GridLayout>
</LinearLayout>

score 0 · Answer 2 · answered Mar 24 '20 at 22:50

Replace your GridLayout in your xml with this

Problem: this R.id.mainGrid wasn't found in your xml, hence the null pointer exception

Solution Add the id android:id="@+id/mainGrid" to your Layout view

<GridLayout
        android:id="@+id/mainGrid"
        android:columnCount="2"
        android:rowCount="3"
        android:alignmentMode="alignMargins"
        android:columnOrderPreserved="false"
        android:layout_weight="8"
        android:padding="14dp"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        >

You can further educate yourself on null pointers here What is a NullPointerException, and how do I fix it?

Cheers!

This solution just works... Thank you very much ! – mtomek95 Mar 26 '20 at 14:16 — mtomek95, Mar 26 '20 at 14:16

Why my app crashes with NullPointerException

2 Answers2