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Editorial Team
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Asked: 2026-06-18T12:59:08+00:00

I have a problem implementing a button listener in a separate class. The code:

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I have a problem implementing a button listener in a separate class. The code:

<Button 
    android:id ="@+button/bottone"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text = "Button"
    />

the main class:

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;
}

and the implementation of listener

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();
}

Now when i try to run it, application crash on my phone, and logcat tolds

unable to instantiate activity componentinfo
on mainactivity.

what’s wrong?

the log of logcat info

02-07 11:43:54.887: E/AndroidRuntime(9048): FATAL EXCEPTION: main
02-07 11:43:54.887: E/AndroidRuntime(9048): java.lang.RuntimeException: Unable to instantiate activity 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): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
02-07 11:43:54.887: E/AndroidRuntime(9048): at android.app.ActivityThread.access$600(ActivityThread.java:141)
02-07 11:43:54.887: E/AndroidRuntime(9048): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
02-07 11:43:54.887: E/AndroidRuntime(9048): at android.os.Handler.dispatchMessage(Handler.java:99)
02-07 11:43:54.887: E/AndroidRuntime(9048): at android.os.Looper.loop(Looper.java:137)
02-07 11:43:54.887: E/AndroidRuntime(9048): at android.app.ActivityThread.main(ActivityThread.java:5039)
02-07 11:43:54.887: E/AndroidRuntime(9048): at java.lang.reflect.Method.invokeNative(Native Method)
02-07 11:43:54.887: E/AndroidRuntime(9048): at java.lang.reflect.Method.invoke(Method.java:511)
02-07 11:43:54.887: E/AndroidRuntime(9048): at 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 by: 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): 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): at android.app.Instrumentation.newActivity(Instrumentation.java:1054)
02-07 11:43:54.887: E/AndroidRuntime(9048): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097)
02-07 11:43:54.887: E/AndroidRuntime(9048): … 11 more

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1 Answer

  1. Editorial Team

    move setContentView(R.layout.activity_main); line before accessing button from UI as :

    public class MainActivity extends Activity {

private  Context main_context; 

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
     setContentView(R.layout.activity_main);  //<< set Activity layout here

    main_context = this.getApplicationContext(); 
    Button b = (Button)findViewById(R.id.bottone);
    Log.i("AAAAAAAAAAAA", "1");

    OnClickListener l = new MyClickListener(this);
    b.setOnClickListener(l);
    Log.i("AAAAAAAAAAAA", "2");



}
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