java.lang.RuntimeException: Unable to start activity

Question

when i try to luch my application, this error message appear, please help:

this is the error message:

   02-21 10:46:40.551    1092-1092/com.example.myapplication3.app E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.myapplication3.app, PID: 1092
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication3.app/com.example.myapplication3.app.MainActivity2}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
        at android.app.ActivityThread.access$700(ActivityThread.java:135)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4998)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at com.example.myapplication3.app.MainActivity2.onCreate(MainActivity2.java:27)
        at android.app.Activity.performCreate(Activity.java:5243)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
        at android.app.ActivityThread.access$700(ActivityThread.java:135)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4998)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
        at dalvik.system.NativeStart.main(Native Method)

the line of java code that is associated to this was an error is: ((Button)findViewById(R.id.monBouton)).setOnClickListener(....)

this my code java:

package com.example.myapplication3.app;



public class MainActivity2 extends ActionBarActivity {

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

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
    ((Button)findViewById(R.id.monBouton)).setOnClickListener(
            new OnClickListener() {
        @Override
        public void onClick(View v) {

                        EditText texte = ((EditText)findViewById(R.id.monEditText));

                String nom = texte.getText().toString();

                Toast.makeText(MainActivity2.this, nom, Toast.LENGTH_SHORT).show();
        }
    });
    }


@Override
public boolean onCreateOptionsMenu(Menu menu) {


    getMenuInflater().inflate(R.menu.main_activity2, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {

    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_main2, container, false);
        return rootView;
    }
}

}

code XML

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">

<EditText
    android:id="@+id/monEditText"
    android:layout_height="wrap_content"
    android:hint="Taper votre nom"
    android:layout_width="fill_parent">

</EditText>
<!--
Le Bouton qui permettra à l’utilisateur
de valider sa saisie
-->
<Button
    android:id="@+id/monBouton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Valider"
    android:layout_gravity="center_horizontal">
</Button>
</LinearLayout>

Answer 1

You are getting a NullpointerException because your layout file does not contain a View (most likely a FrameLayout) with the id R.id.container.

So this piece of code will crash:

getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();

Because "R.id.container" does not exist in the layout file. Put a FrameLayout with the id "container" in your layout file. For example like that:

<FrameLayout
       android:id="@+id/container"
       android:layout_width="match_parent"
       android:layout_height="match_parent" >
</FrameLayout>

Answer 2

Use:

Button mButton = (Button)findViewById(R.id.monBouton); 

Then:

mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                        //Code here
                    }
 });