java - Android - Java - Buttonclicker nullpointerException

Question

我想做一个按钮点击器。当我按下 btnKopen 时，我希望 textView 'BonnenMand' 显示按钮被点击的次数。

这是我为此使用的代码:

Button btnKopen;
TextView bonnenMand;
int bonnenMandCounter = 0;    

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

    Button btnKopen = (Button) findViewById(R.id.buttonKopen);
    final TextView bonnenMand = (TextView) findViewById(R.id.textViewItemCount);  

    btnKopen.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            bonnenMandCounter++;
            bonnenMand.setText(bonnenMandCounter);
        }
    });
}

我收到 NullPointerException 并且应用程序在打开之前崩溃了。

01-04 14:02:48.649 13685-13685/? I/art: Late-enabling -Xcheck:jni
01-04 14:02:48.704 13685-13685/? W/System: ClassLoader referenced unknown path: /data/app/be.ehb.dt.bestellingen-1/lib/arm
01-04 14:02:48.774 13685-13685/? D/AndroidRuntime: Shutting down VM
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime: FATAL EXCEPTION: main
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime: Process: be.ehb.dt.bestellingen, PID: 13685
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{be.ehb.dt.bestellingen/be.ehb.dt.bestellingen.Winkel}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread.-wrap11(ActivityThread.java)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:102)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:148)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5417)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at be.ehb.dt.bestellingen.Winkel.onCreate(Winkel.java:79)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.Activity.performCreate(Activity.java:6251)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:102) 
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:148) 
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5417) 
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method) 
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
01-04 14:02:48.774 13685-13685/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
01-04 14:02:51.469 13685-13685/? I/Process: Sending signal. PID: 13685 SIG: 9

编辑:我的 main_activity.xml 中不存在该按钮，使用以下 arrayAdapter 添加按钮..

public class MenuAdapter extends ArrayAdapter<MenuWinkel> {

        public MenuAdapter(Context context, ArrayList<MenuWinkel> menus) {
            super(context, 0, menus);
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {

            MenuWinkel menu = getItem(position);

            if (convertView == null) {
                convertView = LayoutInflater.from(getContext()).inflate(R.layout.winkelrow, parent, false);
            }

            TextView menuName = (TextView) convertView.findViewById(R.id.textViewMenu);
            TextView menuPrice = (TextView) convertView.findViewById(R.id.textViewPrijs);

            menuName.setText(menu.getNaam());
            if(menu.getPrijs() > 0.0) {
                StringTokenizer tokens = new StringTokenizer(menu.getPrijs()+"", ".");
                String first = tokens.nextToken();// this will contain "Fruit"
                String second = tokens.nextToken();

                    if (second.equals("0")) {
                        menuPrice.setText(first);
                    } else {
                        menuPrice.setText(first + "," + second);
                    }

            }

            return convertView;
        }
    }

这使我能够放置如下项目:

 ArrayList<MenuWinkel> arrayOfMenus = new ArrayList<MenuWinkel>();
        MenuAdapter menuAdapter = new MenuAdapter(this,arrayOfMenus);

 MenuWinkel menu1 = new MenuWinkel("10% korting op drank bij menu",50.0);
        menuAdapter.add(menu1);
        MenuWinkel menu2 = new MenuWinkel("30% korting op drank bij menu",70.0);
        menuAdapter.add(menu2);
        MenuWinkel menu3 = new MenuWinkel("70% korting op drank bij menu",120.0);

此数组适配器添加具有以下 XML 的项目:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="106.5dp"
        android:id="@+id/imageBackground"
        android:src="@drawable/banner_wit"
        android:layout_marginTop="4.5dp"
        android:contentDescription="@string/banner_white"
        android:scaleType="fitXY"/>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:weightSum="1">

    <ImageView
        android:layout_width="0dp"
        android:layout_height="85dp"
        android:id="@+id/imageViewMenu"
        android:src="@drawable/drinks"
        android:contentDescription="Drinks"
        android:layout_weight="0.28"
        android:layout_marginTop="13dp"
        android:layout_marginLeft="-2dp" />

        <TextView
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:text="New Text"
            android:id="@+id/textViewMenu"
            android:layout_gravity="center"
            android:textColor="#1D1D1B"
            android:layout_weight="0.65"
            android:paddingLeft="10dp"
            android:layout_marginTop="0dp"
            android:layout_marginBottom="10dp"/>

        <ImageView
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:id="@+id/imageCaps"
            android:src="@drawable/caps"
            android:layout_weight="0.01"
            android:layout_marginTop="30dp" />

        <TextView
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:id="@+id/textViewPrijs"
            android:layout_gravity="center_vertical"
            android:textColor="#fff"
            android:layout_weight="0.10"
            android:layout_marginLeft="-35dp"
            android:layout_marginTop="8dp" />

    </LinearLayout>

    <Button
        android:layout_width="100dp"
        android:layout_height="30dp"
        android:text="Kopen"
        android:id="@+id/buttonKopen"
        android:textColor="#fff"
        android:background="@drawable/borderradiuskopen"
        android:layout_marginBottom="20dp"
        android:layout_marginLeft="40dp"
        android:layout_gravity="center_horizontal|bottom"
        />
</FrameLayout>

在此 XML 中，您可以找到 buttonKopen 的 buttonID。

Answer 1

你的错误是

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

这是由这些行引起的:

Button btnKopen = (Button) findViewById(R.id.buttonKopen);
btnKopen.setOnClickListener(...);

这是因为activity_main.xml中没有包含id为R.id.buttonKopen的按钮

要向按钮添加点击监听器，请在 MenuAdapter 的 getView 方法中执行此操作。