java - findviewbyid 处的空指针异常

Question

我正在尝试构建一个简单的应用程序，它从我的电脑下载一个 png 文件，对其进行解码并将其设置在按下按钮后启动的新 Activity 中的 ImageView 中。

与网络有关的所有内容以及文件的解码似乎都能正常工作，但是在使用“findviewbyid”时我似乎遇到了很多错误，(android studio)它建议了正确的ID，但是当我运行它，抛出异常并且应用程序崩溃。

(在 android studio 中创建新 Activity 时，它会创建一个占位符 fragment 。我应该在 fragment 的 OncreateView 中编写代码吗？)

Activity XML:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.app.show"
tools:ignore="MergeRootFrame" />

XML fragment :

<RelativeLayout 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:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="com.example.app.show$PlaceholderFragment">

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/fromaster"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true" />
</RelativeLayout>

java代码(onCreate):

public class show extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_show);
    ImageView v;
    v = (ImageView) findViewById(R.id.fromaster);
    byte[] raw;

    if (v == null){Log.d("at view","empty");}

    raw = getIntent().getByteArrayExtra("pic");

    Bitmap bitmap = BitmapFactory.decodeByteArray(raw,0,raw.length);
    if (bitmap == null){
        Toast.makeText(getApplicationContext(),"empty      bitmap",Toast.LENGTH_SHORT).show();
    }
    else{
        try{
            v.setImageBitmap(bitmap);}
        catch (Exception e){
            Log.d("at setting bitmap","error",e);
        }
    }


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

日志猫:

02-02 10:48:34.330  21950-21950/com.example.app E/FragmentManager﹕ No view found for id 0x7f080000 (com.example.app:id/container) for fragment PlaceholderFragment{41bd5c38 #0 id=0x7f080000}

编辑:我已经使用了 fragment 的OncreateView，但使用findviewbyid时仍然出现异常:

java代码:

 public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_show, container, false);

        ImageView v = (ImageView) getView().findViewById(R.id.fromaster);
        byte[] raw;

        if (v == null){Log.d("at view","empty");}

        raw = getArguments().getByteArray("pic");

        Bitmap bitmap = BitmapFactory.decodeByteArray(raw,0,raw.length);
        if (bitmap == null){
            Toast.makeText(getActivity().getApplicationContext(),"empty bitmap",Toast.LENGTH_SHORT).show();
        }
        else{
            try{
                v.setImageBitmap(bitmap);}
            catch (Exception e){
                Log.d("at setting bitmap","error",e);
            }
        }

        return rootView;
    }
}

logcat错误:

02-02 11:29:24.335  25479-25479/com.example.app E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.app, PID: 25479
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.app/com.example.app.show}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2215)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2265)
        at android.app.ActivityThread.access$800(ActivityThread.java:145)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1206)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5081)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:781)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at com.example.app.show$PlaceholderFragment.onCreateView(show.java:69)
        at android.app.Fragment.performCreateView(Fragment.java:1700)
        at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:890)
        at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1062)
        at android.app.BackStackRecord.run(BackStackRecord.java:684)
        at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1447)
        at android.app.Activity.performStart(Activity.java:5240)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2178)

           

Answer 1

activity_show.xml 没有 ImageView

ImageView 位于 fragment.xml 中。 findViewById 在当前展开的布局中查找 View 。因此，如果您 findViewById 返回 null，那么您引用了错误的 View ID，或者您在扩充的 xml 中没有该 View 。

我的猜测是您的 Activity 有一个框架布局，它是您添加或替换框架的容器。 ImageView 的初始化应该转到 Fragment 的 onCreateView ，并且在您的 show Activity 中，您需要将 fragment 添加到 FrameLayout >.

编辑:

View rootView = inflater.inflate(R.layout.fragment_show, container, false);
ImageView v = (ImageView) rootView.findViewById(R.id.fromaster);