为什么我的应用由于NullPointerException而崩溃

问题描述 投票：0回答：2

我尝试使用CardView创建简单的应用。这是我使用android和java的第一步，因此对于您来说，这个问题可能很容易解决。好的，那么，当我尝试在AVD上打开应用时，它崩溃了。

代码：

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);

            }
        });
    }
}

和问题：

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：正在发送信号。 PID：10136 SIG：9

我的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>

这是我的第一篇文章，请不要刻薄：）

java android xml nullpointerexception cardview
2个回答
0
投票

我的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>
0
投票

用此替换XML中的GridLayout

问题：在您的xml中找不到此R.id.mainGrid，因此存在空指针异常

解决方案将ID android:id="@+id/mainGrid"添加到布局视图中

<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"
        >

您可以在What is a NullPointerException, and how do I fix it?处进一步学习空指针的知识>

干杯！

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