java - findViewById 在 onCreate 方法中给出 NullPointer 异常

Question

我在使用 findViewbyId 方法之前放置了 setContentView，但它仍然给我错误。谁能解释一下为什么会发生这种情况。这是我的代码。

public class DispActivity extends ActionBarActivity {

public String message;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_disp);

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

    Intent intent = getIntent();
    message = intent.getStringExtra(MainActivity.Extra);
    EditText txt = (EditText)findViewById(R.id.edit_text23);
    System.out.println("check point 1");
    txt.setText("Having problem here");
    System.out.println("check point 2");


}

fragment_disp.xml 看起来像这样。

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

<EditText 
    android:id="@+id/edit_text23"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
/>
<TextView
    android:id="@+id/text"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />
<Button android:id="@+id/but"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="click"/>
</LinearLayout>

发生的错误是

05-13 23:50:46.716: I/System.out(5713): check point 1
05-13 23:50:46.716: D/AndroidRuntime(5713): Shutting down VM
05-13 23:50:46.716: W/dalvikvm(5713): threadid=1: thread exiting with uncaught     exception (group=0x419c9ba8)
05-13 23:50:46.726: E/AndroidRuntime(5713): FATAL EXCEPTION: main
05-13 23:50:46.726: E/AndroidRuntime(5713): Process: com.example.wifi, PID: 5713
05-13 23:50:46.726: E/AndroidRuntime(5713): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.wifi/com.example.wifi.DispActivity}: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
05-13 23:50:46.726: E/AndroidRuntime(5713):     at  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.os.Looper.loop(Looper.java:136)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at java.lang.reflect.Method.invokeNative(Native Method)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at java.lang.reflect.Method.invoke(Method.java:515)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at dalvik.system.NativeStart.main(Native Method)
05-13 23:50:46.726: E/AndroidRuntime(5713): Caused by: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
05-13 23:50:46.726: E/AndroidRuntime(5713):     at com.example.wifi.DispActivity$PlaceholderFragment.onCreateView(DispActivity.java:74)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.Fragment.performCreateView(Fragment.java:1500)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:938)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1115)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1478)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:570)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.Activity.performStart(Activity.java:5241)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2168)
05-13 23:50:46.726: E/AndroidRuntime(5713):     ... 11 more

Answer 1

您正在尝试将 TextView 转换为 EditText，这会导致错误。你确定你已经给出了完整的相关代码吗？堆栈跟踪非常清晰，带有行号。