Question

即使我使用其他类（如编辑文本），我也遇到此问题。

我确信id。

这是代码：

public class MainActivity extends ActionBarActivity 
{
    @Override
    protected void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button bshow = (Button)findViewById(R.id.show_msg);

        bshow.setText("Showing");
    }

}

这是fragment_main.xml

    <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="com.exampleknight283.e003.MainActivity$PlaceholderFragment" >

   <TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

   <Button
    android:id="@+id/show_msg"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_marginTop="44dp"
    android:text="@string/showv" />

    </RelativeLayout>

我在退出时得到了这个

  07-16 11:27:32.155: E/Trace(1382): error opening trace file: No such file or      directory (2)
  07-16 11:27:32.625: D/AndroidRuntime(1382): Shutting down VM
  07-16 11:27:32.625: W/dalvikvm(1382): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
  07-16 11:27:32.645: E/AndroidRuntime(1382): FATAL EXCEPTION: main
  07-16 11:27:32.645: E/AndroidRuntime(1382): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.exampleknight283.e003/com.exampleknight283.e003.MainActivity}: java.lang.NullPointerException
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at android.app.ActivityThread.access$600(ActivityThread.java:141)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at android.os.Handler.dispatchMessage(Handler.java:99)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at android.os.Looper.loop(Looper.java:137)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at android.app.ActivityThread.main(ActivityThread.java:5041)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at java.lang.reflect.Method.invokeNative(Native Method)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at java.lang.reflect.Method.invoke(Method.java:511)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
  07-16 11:27:32.645: E/AndroidRuntime(1382):   at dalvik.system.NativeStart.main(Native Method)
  07-16 11:27:32.645: E/AndroidRuntime(1382): Caused by: java.lang.NullPointerException
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at com.exampleknight283.e003.MainActivity.onCreate(MainActivity.java:29)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at android.app.Activity.performCreate(Activity.java:5104)
 07-16 11:27:32.645: E/AndroidRuntime(1382):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
  07-16 11:27:32.645: E/AndroidRuntime(1382):   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)

我是android dev的新手。但我在java方面很好，现在我遵循一些教程，我做得非常好，但是我有错误，就像我说的那样，我和其他类有同样的问题，比如＆＃34;编辑班＆＃34;

Answer 1

尝试：

public class MainActivity extends ActionBarActivity 
{
    Button bshow;

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

        bshow = (Button) findViewById(R.id.show_msg);

        bshow.setText("Showing");
    }

}

不幸的是app已停止按钮对象

1 个答案: