当在Activity中访问小部件时发生NullPointerException异常

Question

无论我将哪个代码作为主要活动运行，我的LogCat都会显示NullPointerException。在同一个或不同项目中尝试的所有活动中都会发生同样的例外情况。

MainActivity

public class MainActivity extends Activity implements View.OnClickListener{ 
    private static EditText et; 
    private static Button b; 
    private static TextView tv; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     et=(EditText)findViewById(R.id.editText1); 
     Button b=(Button)findViewById(R.id.button1); 
     TextView tv=(TextView)findViewById(R.id.textView1); 
     b.setOnClickListener(this); 

     setContentView(R.layout.activity_main); 
    } 

    @Override 
    public void onClick(View v) { 
     // TODO Auto-generated method stub 
     tv.setText(et.getText()); 
    } 
} 

activity_main.xml中

<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: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=".MainActivity" > 

    <EditText 
     android:id="@+id/editText1" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_alignParentTop="true" 
     android:layout_centerHorizontal="true" 
     android:layout_marginTop="73dp" 
     android:ems="10" > 

     <requestFocus /> 
    </EditText> 

    <Button 
     android:id="@+id/button1" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/editText1" 
     android:layout_centerHorizontal="true" 
     android:layout_marginTop="68dp" 
     android:text="Button" /> 

    <TextView 
     android:id="@+id/textView1" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/button1" 
     android:layout_centerHorizontal="true" 
     android:layout_marginTop="72dp" 
     android:text="TextView" /> 

</RelativeLayout> 

清单文件

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
    package="com.android.stuff" 
    android:versionCode="1" 
    android:versionName="1.0" > 

    <uses-sdk 
     android:minSdkVersion="8" 
     android:targetSdkVersion="15" /> 

    <application 
     android:allowBackup="true" 
     android:icon="@drawable/ic_launcher" 
     android:label="@string/app_name" 
     android:theme="@style/AppTheme" > 
     <activity 
      android:name="com.android.stuff.MainActivity" 
      android:label="@string/app_name" > 
      <intent-filter> 
       <action android:name="android.intent.action.MAIN" /> 

       <category android:name="android.intent.category.LAUNCHER" /> 
      </intent-filter> 
     </activity> 
    </application> 
</manifest> 

logcat的

10-09 05:52:58.953: E/Trace(667): error opening trace file: No such file or directory (2) 
10-09 05:53:00.152: D/AndroidRuntime(667): Shutting down VM 
10-09 05:53:00.172: W/dalvikvm(667): threadid=1: thread exiting with uncaught exception (group=0x40a13300) 
10-09 05:53:00.202: E/AndroidRuntime(667): FATAL EXCEPTION: main 
10-09 05:53:00.202: E/AndroidRuntime(667): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android.stuff/com.android.stuff.MainActivity}: java.lang.NullPointerException 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059) 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.app.ActivityThread.access$600(ActivityThread.java:130) 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.os.Handler.dispatchMessage(Handler.java:99) 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.os.Looper.loop(Looper.java:137) 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.app.ActivityThread.main(ActivityThread.java:4745) 
10-09 05:53:00.202: E/AndroidRuntime(667): at java.lang.reflect.Method.invokeNative(Native Method) 
10-09 05:53:00.202: E/AndroidRuntime(667): at java.lang.reflect.Method.invoke(Method.java:511) 
10-09 05:53:00.202: E/AndroidRuntime(667): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
10-09 05:53:00.202: E/AndroidRuntime(667): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
10-09 05:53:00.202: E/AndroidRuntime(667): at dalvik.system.NativeStart.main(Native Method) 
10-09 05:53:00.202: E/AndroidRuntime(667): Caused by: java.lang.NullPointerException 
10-09 05:53:00.202: E/AndroidRuntime(667): at com.android.stuff.MainActivity.onCreate(MainActivity.java:23) 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.app.Activity.performCreate(Activity.java:5008) 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079) 
10-09 05:53:00.202: E/AndroidRuntime(667): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023) 
10-09 05:53:00.202: E/AndroidRuntime(667): ... 11 more 
10-09 05:53:03.652: I/Process(667): Sending signal. PID: 667 SIG: 9 

Answer 1

使用此在您的onCreate方法：

super.onCreate(savedInstanceState); 
setContentView(R.layout.activity_main); 

Answer 2

设置访问组件之前，内容视图。另外，请勿在onCreate()中再次声明该类型，因为它们将成为遮蔽静态变量的局部变量。

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

    et=(EditText)findViewById(R.id.editText1); 
    b=(Button)findViewById(R.id.button1); 
    tv=(TextView)findViewById(R.id.textView1); 

    b.setOnClickListener(this); 
}