Crash when adding a OnClickListener to a Button

Question

I'm a total newbie with Android, and I'm trying to switch between two activities ("MainActivity" and "LoginDisplayActivity") and adding the "OnClickListener" seem to make my app crash on startup.

Here is my code :

package info.dremor.kronos;

import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.os.Build;

public class MainActivity extends Activity {

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

        if(savedInstanceState == null) {
            getFragmentManager().beginTransaction().add(R.id.container, new MainFragment()).commit();
        }

        final Button loginButton = (Button) findViewById(R.id.connect);
        loginButton.setOnClickListener(new OnClickListener() {  
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, LoginDisplayActivity.class);
                startActivity(intent);
            }
        }); 
    }

    @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 MainFragment extends Fragment {

        public MainFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
    }
}

trace :

05-26 15:48:11.286: D/AndroidRuntime(947): Shutting down VM
05-26 15:48:11.286: W/dalvikvm(947): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
05-26 15:48:11.296: E/AndroidRuntime(947): FATAL EXCEPTION: main
05-26 15:48:11.296: E/AndroidRuntime(947): java.lang.RuntimeException: Unable to start activity ComponentInfo{info.dremor.kronos/info.dremor.kronos.MainActivity}: java.lang.NullPointerException
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.app.ActivityThread.access$600(ActivityThread.java:130)
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.os.Handler.dispatchMessage(Handler.java:99)
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.os.Looper.loop(Looper.java:137)
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.app.ActivityThread.main(ActivityThread.java:4745)
05-26 15:48:11.296: E/AndroidRuntime(947):  at java.lang.reflect.Method.invokeNative(Native Method)
05-26 15:48:11.296: E/AndroidRuntime(947):  at java.lang.reflect.Method.invoke(Method.java:511)
05-26 15:48:11.296: E/AndroidRuntime(947):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
05-26 15:48:11.296: E/AndroidRuntime(947):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
05-26 15:48:11.296: E/AndroidRuntime(947):  at dalvik.system.NativeStart.main(Native Method)
05-26 15:48:11.296: E/AndroidRuntime(947): Caused by: java.lang.NullPointerException
05-26 15:48:11.296: E/AndroidRuntime(947):  at info.dremor.kronos.MainActivity.onCreate(MainActivity.java:31)
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.app.Activity.performCreate(Activity.java:5008)
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
05-26 15:48:11.296: E/AndroidRuntime(947):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
05-26 15:48:11.296: E/AndroidRuntime(947):  ... 11 more

activity_main.xml :

<FrameLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="info.dremor.kronos.MainActivity"
    tools:ignore="MergeRootFrame" />

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="info.dremor.kronos.MainActivity$PlaceholderFragment" >

    <Button
        android:id="@+id/connect"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="35dp"
        android:text="@string/connect" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="35dp"
        android:text="@string/login_greeting" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/textView1"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="35dp"
        android:text="@string/login" />

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/textView3"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="35dp"
        android:ems="10" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/editText1"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="35dp"
        android:text="@string/password" />

    <EditText
        android:id="@+id/editText2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/editText1"
        android:layout_below="@+id/textView2"
        android:layout_marginTop="35dp"
        android:ems="10"
        android:inputType="textPassword" />

</RelativeLayout>

Can someone help me?

Answer 1

Remove the button and code from onCreate and shift the button and clicklistener code to onCreateView like this:

View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
     final Button loginButton = (Button) rootView.findViewById(R.id.connect);
              loginButton.setOnClickListener(new OnClickListener() {

              @Override
              public void onClick(View v) {
                  Intent intent = new Intent(MainActivity.this, LoginDisplayActivity.class);
                  startActivity(intent);
                }
            });
return rootView;  

As your button is in fragment_main, not activity_main. The button is not found.

Answer 2

In your activity, you set the layout file like this:

setContentView(R.layout.activity_main);

This means, when you write findViewById, it will look within the activity_main.xml file.

Your button on the other hand is within another layout file.

Just move the click event into your fragment, and access the context using getActivity like so:

final Button loginButton = (Button) rootView.findViewById(R.id.connect);
loginButton.setOnClickListener(new OnClickListener() {

    @Override
    public void onClick(View v) {
        Intent intent = new Intent(getActivity(), LoginDisplayActivity.class);
        startActivity(intent);
    }
});

Answer 3

your button is in the fragment layout, you're "finding" it from the activity layout. Setup button onclick in the fragment onCreateView

 @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            final Button loginButton = (Button) rootView.findViewById(R.id.connect);
            loginButton.setOnClickListener(new OnClickListener(){...}) 
            return rootView;
        }