setOnClickListener使我的应用程序崩溃[重复]

Question

这个问题已经在这里有了答案：

• NullPointerException访问onCreate（）中的视图 13个答案

我知道这个问题已经被问过很多次了，但是其他答案都没有帮助我。 当我注释掉setOnClickListener时，我的代码可以正常工作。

所以，这是我的主要活动...

public class StartingPoint extends ActionBarActivity {

int ctr = 0;
Button add, sub;
TextView display;

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

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }

    //find views
    display = (TextView) findViewById(R.id.tvTotal);
    add = (Button)this. findViewById(R.id.bAdd);
    sub = (Button)this. findViewById(R.id.bSub);

    //click listeners
    OnClickListener lis = new OnClickListener() {
        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            ctr++;
            display.setText("The total is " + ctr);
        }
    };
    add.setOnClickListener(lis);

    /**sub.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            ctr--;
            display.setText("The total is " + ctr);
        }
    });*/

}
@Override
public boolean onCreateOptionsMenu(Menu menu) {...}

@Override
public boolean onOptionsItemSelected(MenuItem item) {...}

public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_starting_point,
                container, false);
        return rootView;
    }
}

}

add.setOnClickListener（lis）; 使应用程序崩溃。

这是fragment_starting_point.xml ...

<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"
android:orientation="vertical" >

<TextView
    android:id="@+id/tvTotal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="The total is 0"
    android:layout_gravity="top"
    android:gravity="center"
    android:textSize="30sp"
    android:textColor="@color/red"
/>

<Button
    android:id="@+id/bAdd"
    android:layout_width="150sp"
    android:layout_height="wrap_content"
    android:text="Add 1"
    android:textSize="20sp"
    android:layout_gravity="center"
    android:gravity="center"
    />

<Button
    android:id="@+id/bSub"
    android:layout_width="150sp"
    android:layout_height="wrap_content"
    android:text="Subtract 1"
    android:textSize="20sp"
    android:layout_gravity="center"
    android:gravity="center"
/>

这就是logCat所说的...

05-19 23:23:09.700: D/CLIPBOARD(16417): Hide Clipboard dialog at Starting input:         finished by someone else... !
05-19 23:23:34.085: I/dalvikvm(16518): Could not find method     android.widget.LinearLayout$LayoutParams.<init>, referenced from method     android.support.v7.internal.view.menu.ActionMenuView$LayoutParams.<init>
05-19 23:23:34.085: W/dalvikvm(16518): VFY: unable to resolve direct method 8208:    Landroid/widget/LinearLayout$LayoutParams;.<init> (Landroid/widget/LinearLayout$LayoutParams;)V
05-19 23:23:34.085: D/dalvikvm(16518): VFY: replacing opcode 0x70 at 0x0000
05-19 23:23:34.115: D/CLIPBOARD(16518): Hide Clipboard dialog at Starting input: finished by someone else... !
05-19 23:23:37.760: D/CLIPBOARD(16518): Hide Clipboard dialog at Starting input: finished by someone else... !
05-19 23:28:52.000: D/AndroidRuntime(16834): Shutting down VM
05-19 23:28:52.000: W/dalvikvm(16834): threadid=1: thread exiting with uncaught exception (group=0x40192760)
05-19 23:28:52.000: E/AndroidRuntime(16834): FATAL EXCEPTION: main
05-19 23:28:52.000: E/AndroidRuntime(16834): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example2.mysecondapp/com.example2.mysecondapp.StartingPoint}: java.lang.NullPointerException
05-19 23:28:52.000: E/AndroidRuntime(16834):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1815)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at android.app.ActivityThread.access$500(ActivityThread.java:122)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at android.os.Handler.dispatchMessage(Handler.java:99)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at android.os.Looper.loop(Looper.java:132)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at android.app.ActivityThread.main(ActivityThread.java:4123)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at java.lang.reflect.Method.invokeNative(Native Method)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at java.lang.reflect.Method.invoke(Method.java:491)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:844)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:602)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at dalvik.system.NativeStart.main(Native Method)
05-19 23:28:52.000: E/AndroidRuntime(16834): Caused by: java.lang.NullPointerException
05-19 23:28:52.000: E/AndroidRuntime(16834):    at com.example2.mysecondapp.StartingPoint.onCreate(StartingPoint.java:47)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1050)
05-19 23:28:52.000: E/AndroidRuntime(16834):    at     android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1779)
05-19 23:28:52.000: E/AndroidRuntime(16834):    ... 11 more
05-19 23:28:59.720: I/Process(16834): Sending signal. PID: 16834 SIG: 9

我不知道到底需要什么，所以我只是发布了我认为相关的所有内容，对此我还是很陌生。 我真的很感谢您的帮助。

Answer 1

Fragment的布局包含ID为R.id.bAdd的Button ，而Activity的布局则没有。 这是错误的根源，此行：

add = (Button)this.findViewById(R.id.bAdd);

在Activity的布局中找不到Button ，因此它返回null。 之后，当您尝试设置OnClickListener将收到NullPointerException 。

您需要做的是将与Buttons等的所有交互都移到Fragment它不属于Activity因为它不是Activity布局的一部分。 覆盖Fragment的onCreateView()方法，然后在其中填充布局，然后查找如下所示的Views ：

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_starting_point, container, false);

    this.add = (Button) view.findViewById(R.id.bAdd);

    return view;
}

然后，您可以在onViewCreated()添加OnClickListener ：

@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);

    this.add.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            ...    
        }
    });
}

Answer 2

这是因为当按钮位于片段的布局fragment_main ， findViewById()在activity_main布局中进行搜索。

在片段的onCreateView()方法中移动这段代码：

//...

View rootView = inflater.inflate(R.layout.fragment_main, container, false);
Button buttonClick = (Button)rootView.findViewById(R.id.button);
buttonClick.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        onButtonClick((Button) view);
    }
});

注意，现在您可以通过rootView view访问它：

Button buttonClick = (Button)rootView.findViewById(R.id.button);

否则，您将再次获得NullPointerException 。