我在单独的类中实现按钮侦听器时遇到问题。编码:
<Button
android:id ="@+button/bottone"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text = "Button"
/>
主要课程:
public class MainActivity extends Activity {
private final Context main_context = this.getApplicationContext();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Button b = (Button)findViewById(R.button.bottone);
Log.i("AAAAAAAAAAAA", "1");
OnClickListener l = new MyClickListener(this);
b.setOnClickListener(l);
Log.i("AAAAAAAAAAAA", "2");
setContentView(R.layout.activity_main);
}
public Context getContext() {
return main_context;
}
和监听器的实现
public class MyClickListener implements OnClickListener {
private final Context cntx;
public MyClickListener(Context c) {
cntx = c;
}
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
Log.i("ONCLICK", "bottone cliccato");
Toast t = Toast.makeText(cntx, "OK", Toast.LENGTH_LONG);
t.show();
}
现在当我尝试运行它时,我的手机上的应用程序崩溃了,logcat 告诉我
无法在 mainactivity 上实例化活动组件信息。
怎么了?
logcat 信息的日志
02-07 11:43:54.887: E/AndroidRuntime(9048): 致命异常: main 02-07 11:43:54.887: E/AndroidRuntime(9048): java.lang.RuntimeException: 无法实例化活动 ComponentInfo{com. example.buttoneventlistener/com.example.buttoneventlistener.MainActivity}: java.lang.NullPointerException 02-07 11:43:54.887: E/AndroidRuntime(9048): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2106) 02 -07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app .ActivityThread.access$600(ActivityThread.java:141) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 02-07 11: 43:54.887:E/AndroidRuntime(9048):在 android.os.Handler。dispatchMessage(Handler.java:99) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.os.Looper.loop(Looper.java:137) 02-07 11:43:54.887: E/ AndroidRuntime(9048): 在 android.app.ActivityThread.main(ActivityThread.java:5039) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 java.lang.reflect.Method.invokeNative(Native Method) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 java.lang.reflect.Method.invoke(Method.java:511) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 02-07 11:43:54.887: E/AndroidRuntime(9048): at com.android.internal.os.ZygoteInit.main(ZygoteInit .java:560) 02-07 11:43:54.887: E/AndroidRuntime(9048): at dalvik.system.NativeStart.main(Native Method) 02-07 11:43:54.887: E/AndroidRuntime(9048): Caused作者:java.lang。NullPointerException 02-07 11:43:54.887: E/AndroidRuntime(9048): at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:109) 02-07 11:43:54.887: E/AndroidRuntime(9048): at com .example.buttoneventlistener.MainActivity.(MainActivity.java:13) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 java.lang.Class.newInstanceImpl(Native Method) 02-07 11:43:54.887 : E/AndroidRuntime(9048): at java.lang.Class.newInstance(Class.java:1319) 02-07 11:43:54.887: E/AndroidRuntime(9048): at android.app.Instrumentation.newActivity(Instrumentation. java:1054) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097) 02-07 11:43:54.887: E/AndroidRuntime(9048) : ... 11 更多content.ContextWrapper.getApplicationContext(ContextWrapper.java:109) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 com.example.buttoneventlistener.MainActivity.(MainActivity.java:13) 02-07 11:43 :54.887: E/AndroidRuntime(9048): at java.lang.Class.newInstanceImpl(Native Method) 02-07 11:43:54.887: E/AndroidRuntime(9048): at java.lang.Class.newInstance(Class.java :1319) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.Instrumentation.newActivity(Instrumentation.java:1054) 02-07 11:43:54.887: E/AndroidRuntime(9048):在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097) 02-07 11:43:54.887: E/AndroidRuntime(9048): ... 11 更多content.ContextWrapper.getApplicationContext(ContextWrapper.java:109) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 com.example.buttoneventlistener.MainActivity.(MainActivity.java:13) 02-07 11:43 :54.887: E/AndroidRuntime(9048): at java.lang.Class.newInstanceImpl(Native Method) 02-07 11:43:54.887: E/AndroidRuntime(9048): at java.lang.Class.newInstance(Class.java :1319) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.Instrumentation.newActivity(Instrumentation.java:1054) 02-07 11:43:54.887: E/AndroidRuntime(9048):在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097) 02-07 11:43:54.887: E/AndroidRuntime(9048): ... 11 更多MainActivity.(MainActivity.java:13) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 java.lang.Class.newInstanceImpl(Native Method) 02-07 11:43:54.887: E/AndroidRuntime( 9048): 在 java.lang.Class.newInstance(Class.java:1319) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.Instrumentation.newActivity(Instrumentation.java:1054) 02 -07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097) 02-07 11:43:54.887: E/AndroidRuntime(9048): ... 11更多的MainActivity.(MainActivity.java:13) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 java.lang.Class.newInstanceImpl(Native Method) 02-07 11:43:54.887: E/AndroidRuntime( 9048): 在 java.lang.Class.newInstance(Class.java:1319) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.Instrumentation.newActivity(Instrumentation.java:1054) 02 -07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097) 02-07 11:43:54.887: E/AndroidRuntime(9048): ... 11更多的newActivity(Instrumentation.java:1054) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097) 02-07 11:43:54.887: E/ AndroidRuntime(9048): ... 11 更多newActivity(Instrumentation.java:1054) 02-07 11:43:54.887: E/AndroidRuntime(9048): 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097) 02-07 11:43:54.887: E/ AndroidRuntime(9048): ... 11 更多