我在MainActivity课程中有这段代码：
public class MainActivity extends ActionBarActivity {
Button mButton;
EditText mEdit;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mButton = (Button)findViewById(R.id.btnAsk);
mEdit = (EditText)findViewById(R.id.txtQuestion);
mButton.setOnClickListener(
new View.OnClickListener()
{
public void onClick(View view)
{
Toast.makeText(getApplicationContext(),
mEdit.getText().toString(), Toast.LENGTH_SHORT).show();
}
});
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
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_main, container,
false);
return rootView;
}
}
}
一切都很好，但在setOnClickListener()添加mButton之后，它就开始崩溃了。
以下是错误日志：
04-26 14:12:26.255: E/AndroidRuntime(1447): FATAL EXCEPTION: main
04-26 14:12:26.255: E/AndroidRuntime(1447): Process: com.gelasoft.answeringball, PID: 1447
04-26 14:12:26.255: E/AndroidRuntime(1447): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.gelasoft.answeringball/com.gelasoft.answeringball.MainActivity}: java.lang.NullPointerException
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.os.Handler.dispatchMessage(Handler.java:102)
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.os.Looper.loop(Looper.java:136)
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.app.ActivityThread.main(ActivityThread.java:5017)
04-26 14:12:26.255: E/AndroidRuntime(1447): at java.lang.reflect.Method.invokeNative(Native Method)
04-26 14:12:26.255: E/AndroidRuntime(1447): at java.lang.reflect.Method.invoke(Method.java:515)
04-26 14:12:26.255: E/AndroidRuntime(1447): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-26 14:12:26.255: E/AndroidRuntime(1447): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-26 14:12:26.255: E/AndroidRuntime(1447): at dalvik.system.NativeStart.main(Native Method)
04-26 14:12:26.255: E/AndroidRuntime(1447): Caused by: java.lang.NullPointerException
04-26 14:12:26.255: E/AndroidRuntime(1447): at com.gelasoft.answeringball.MainActivity.onCreate(MainActivity.java:30)
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.app.Activity.performCreate(Activity.java:5231)
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-26 14:12:26.255: E/AndroidRuntime(1447): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-26 14:12:26.255: E/AndroidRuntime(1447): ... 11 more
我在这里缺少什么？我知道它确实很小，但我现在无法发现它。
P.S这是我的片段xml：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.xx.ss.MainActivity$PlaceholderFragment" >
<ImageView
android:id="@+id/sphereIcon"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:contentDescription="@string/magicBallDescr"
android:src="@drawable/ic_launcher" />
<LinearLayout
android:id="@+id/table"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_below="@+id/sphereIcon"
android:orientation="vertical" >
<TextView
android:id="@+id/txtAnswer"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
<EditText
android:id="@+id/txtQuestion"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="@string/textHint"
android:inputType="textMultiLine"
android:lines="8"
/>
<Button
android:id="@+id/btnAsk"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="@string/btnAskQ"
android:paddingEnd="@dimen/activity_vertical_margin"
/>
</LinearLayout>
</RelativeLayout>
答案 0 :(得分：2)
替换
mButton = (Button)findViewById(R.id.btnAsk);
与
mButton = (Button)rootView.findViewById(R.id.btnAsk); // this will go in Fragment onCreateView
将setOnClicklistener代码也移动到onCreateView。
希望它有所帮助。
答案 1 :(得分：1)
移动此代码：
mButton.setOnClickListener(
new View.OnClickListener()
{
public void onClick(View view)
{
Toast.makeText(getApplicationContext(),
mEdit.getText().toString(), Toast.LENGTH_SHORT).show();
}
});
进入片段的onCreateView()方法。它应该工作。