"Unfortunately -app- has stopped" on my fist android app ever

Question

I've been having this issue, this is my first android app and I probably did something stupid. I am a total noob at this and this has been driving me insane. Running the app will give me the error, "Unfortunately -app- has stopped working." Help would be appreciated

Fragment Main:

<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="com.gmail.sherwoodax.appone.MainActivity$PlaceholderFragment" >

    <Button
        android:id="@+id/dBID"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Dead Button" 
        android:layout_centerInParent="true"

        />

    <TextView
        android:id="@+id/tView"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="The Button is below me"
        android:layout_above="@+id/dBID"
        android:gravity="center"
        android:textSize="20sp"

        />

</RelativeLayout>

Main Class:

package com.gmail.sherwoodax.appone;



import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity 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();

            testMethod();
        }



    }


    private void testMethod() {

        Button b = (Button) findViewById(R.id.dBID);

        b.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub

                Toast.makeText(MainActivity.this, "text", Toast.LENGTH_LONG).show();

            }
        });

    }


    @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 PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

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

}

Manifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.gmail.sherwoodax.appone"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="19" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.gmail.sherwoodax.appone.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

LogCat stacktrace:

05-28 11:15:54.977: E/AndroidRuntime(31586): Caused by: java.lang.IllegalArgumentException: No view found for id 0x7f05003c (com.gmail.sherwoodax.appone:id/container) for fragment PlaceholderFragment{41c157b0 #0 id=0x7f05003c}
05-28 11:20:47.344: E/AndroidRuntime(32409): Caused by: java.lang.IllegalArgumentException: No view found for id 0x7f05003c (com.gmail.sherwoodax.appone:id/container) for fragment PlaceholderFragment{41c10d00 #0 id=0x7f05003c}

Thank you

Answer 1

Please post the logcat stacktrace for the crash so we can help pinpoint the exact problem. However, I would guess that it has to do with your Button.

In your testMethod() you declare:

Button b = (Button) findViewById(R.id.dBID);

But because it is declared inside this method it is not within scope for your activity and exists only inside that single method call and is lost afterwards.

You should declare your button as a class level variable in your Activity/Fragment:

public class MainActivity extends ActionBarActivity {
    private Button myButton;
    ...

then assign the Button in your onCreate() (or onViewCreate() for a Fragment):

public void onCreate(...) {
    myButton = (Button) findViewById(...);
    ...

and then anywhere else in your class you want to use that Button you can refer to it without needing to pull and cast it again from your view:

button.setText(newButtonText);