听众的麻烦

Question

I am trying to learn how listeners work in Android. For this I made a simple app with 2 activities where the mainactivity 'listens to' the second activity. The app crashes.

Since the entire app is pretty small, I am going to post the entire thing here along with the logcat.

My question is: what am I doing wrong and how do I get it to work.

The MainActivity:(this listens to the Listen activity)

public class MainActivity extends Activity implements Listen.Listener {

Listen l;

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

            l = new Listen();
    l.setListener(this);

}


public void onButton(final View v)
{
    Intent i = new Intent(this, Listen.class);
    startActivity(i);
}

@Override
public void withoutMessage()
{
    Toast.makeText(this, "click received in the main activity class", Toast.LENGTH_SHORT).show();
}

}

The Listen activity:

public class Listen extends Activity {

  public interface Listener {
      public void withoutMessage();

  }

  Listener listner = null;

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


}

 public void setListener(Listener l) {
     listner = l;
    }

public void onButton(final View v)
{
    Log.w("Rakshak", "button click");
    Toast.makeText(this, "button clicked from the the Listen class", Toast.LENGTH_SHORT).show();
    listner.withoutMessage();

}

}

The LogCat:

05-30 11:20:50.764: E/AndroidRuntime(4726): FATAL EXCEPTION: main
05-30 11:20:50.764: E/AndroidRuntime(4726): java.lang.IllegalStateException: Could not execute method of the activity
05-30 11:20:50.764: E/AndroidRuntime(4726):     at android.view.View$1.onClick(View.java:3607)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at android.view.View.performClick(View.java:4212)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at android.view.View$PerformClick.run(View.java:17476)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at android.os.Handler.handleCallback(Handler.java:800)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at android.os.Handler.dispatchMessage(Handler.java:100)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at android.os.Looper.loop(Looper.java:194)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at android.app.ActivityThread.main(ActivityThread.java:5371)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at java.lang.reflect.Method.invokeNative(Native Method)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at java.lang.reflect.Method.invoke(Method.java:525)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at dalvik.system.NativeStart.main(Native Method)
05-30 11:20:50.764: E/AndroidRuntime(4726): Caused by: java.lang.reflect.InvocationTargetException
05-30 11:20:50.764: E/AndroidRuntime(4726):     at java.lang.reflect.Method.invokeNative(Native Method)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at java.lang.reflect.Method.invoke(Method.java:525)
05-30 11:20:50.764: E/AndroidRuntime(4726):     at  android.view.View$1.onClick(View.java:3602)
05-30 11:20:50.764: E/AndroidRuntime(4726):     ... 11 more
05-30 11:20:50.764: E/AndroidRuntime(4726): Caused by: java.lang.NullPointerException
05-30 11:20:50.764: E/AndroidRuntime(4726):     at com.example.listnertest.Listen.onButton(Listen.java:35)
05-30 11:20:50.764: E/AndroidRuntime(4726):     ... 14 more

Answer 1

This

 l = new Listen();

is wrong since its a Activity class. Never instantiate a Activity class. Activity class is not a normal java class. It has a lifecycle and it is declared in manifest file.

Your listner is null.

Answer 2

You cant just instantiante a Activity class.. thats the error come from.. it will return a null..

solution:

just create a interface and instead of activity you can create a fragment.. which will listen to your main activity