title: Null pointer exception on my code
body: <p>I got all my code set up and completed. I tried running my app to make sure it functions as it should and I get a <strong>Unfortunately your app has stopped</strong>. I tried looking at other examples of this problem but couldn't find out what actually is wrong with my app. I checked my LogCat and still couldn't figure it out. If I could get a hand on what I should look at I would appreciate it. </p>
  
  <pre>
  10-11 22:13:01.521: W/AudioService(286): Soundpool could not load file: /system/media/audio/ui/KeypressReturn.ogg
  10-11 22:13:01.540: W/AudioService(286): onLoadSoundEffects(), Error -1 while loading samples
  10-11 22:13:01.551: I/ActivityManager(286): START u0 {act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10200000 cmp=net.androidbootcamp.zipcarrentalapp/.MainActivity} from pid 414
  10-11 22:13:01.611: W/WindowManager(286): Screenshot failure taking screenshot for (266x425) to layer 21010
  10-11 22:13:01.691: W/dalvikvm(859): PR_CAPBSET_DROP 0 failed: Invalid argument. Please make sure your kernel is compiled with file capabilities support enabled.
  10-11 22:13:01.771: D/dalvikvm(859): Not late-enabling CheckJNI (already on)
  10-11 22:13:01.781: I/ActivityManager(286): Start proc net.androidbootcamp.zipcarrentalapp for activity net.androidbootcamp.zipcarrentalapp/.MainActivity: pid=859 uid=10046 gids={50046, 1028}
  10-11 22:13:03.620: D/AndroidRuntime(859): Shutting down VM
  10-11 22:13:03.650: W/dalvikvm(859): threadid=1: thread exiting with uncaught exception (group=0x41465700)
  10-11 22:13:03.770: E/AndroidRuntime(859): FATAL EXCEPTION: main
  10-11 22:13:03.770: E/AndroidRuntime(859): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{net.androidbootcamp.zipcarrentalapp/net.androidbootcamp.zipcarrentalapp.MainActivity}: java.lang.NullPointerException
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2137)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.app.ActivityThread.access$600(ActivityThread.java:141)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.os.Handler.dispatchMessage(Handler.java:99)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.os.Looper.loop(Looper.java:137)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.app.ActivityThread.main(ActivityThread.java:5103)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at java.lang.reflect.Method.invokeNative(Native Method)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at java.lang.reflect.Method.invoke(Method.java:525)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at dalvik.system.NativeStart.main(Native Method)
  10-11 22:13:03.770: E/AndroidRuntime(859): Caused by: java.lang.NullPointerException
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.app.Activity.findViewById(Activity.java:1853)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at net.androidbootcamp.zipcarrentalapp.MainActivity.(MainActivity.java:16)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at java.lang.Class.newInstanceImpl(Native Method)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at java.lang.Class.newInstance(Class.java:1130)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
  10-11 22:13:03.770: E/AndroidRuntime(859):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2128)
  10-11 22:13:03.770: E/AndroidRuntime(859):  ... 11 more
  10-11 22:13:04.510: W/ActivityManager(286):   Force finishing activity net.androidbootcamp.zipcarrentalapp/.MainActivity
  10-11 22:13:04.983: I/WindowManager(286): Screenshot max retries 4 of Token{41ad2f58 ActivityRecord{41989b98 u0 net.androidbootcamp.zipcarrentalapp/.MainActivity}} appWin=Window{41ab4b70 u0 Starting net.androidbootcamp.zipcarrentalapp} drawState=4
  10-11 22:13:04.983: W/WindowManager(286): Screenshot failure taking screenshot for (800x1280) to layer 21015
  10-11 22:13:05.010: W/ActivityManager(286):   Force finishing activity com.android.customlocale2/.CustomLocaleActivity
  10-11 22:13:05.450: I/Choreographer(414): Skipped 423 frames!  The application may be doing too much work on its main thread.
  10-11 22:13:05.720: W/ActivityManager(286): Activity pause timeout for ActivityRecord{41989b98 u0 net.androidbootcamp.zipcarrentalapp/.MainActivity}
  10-11 22:13:05.970: I/Choreographer(286): Skipped 33 frames!  The application may be doing too much work on its main thread.
  10-11 22:13:06.410: I/Choreographer(414): Skipped 32 frames!  The application may be doing too much work on its main thread.
  10-11 22:13:06.780: I/ActivityManager(286): Activity reported stop, but no longer stopping: ActivityRecord{419b5e78 u0 com.android.launcher/com.android.launcher2.Launcher}
  10-11 22:13:08.680: E/SoundPool(286): error loading /system/media/audio/ui/Effect_Tick.ogg
  10-11 22:13:08.680: W/AudioService(286): Soundpool could not load file: /system/media/audio/ui/Effect_Tick.ogg
  10-11 22:13:08.871: E/SoundPool(286): error loading /system/media/audio/ui/KeypressStandard.ogg
  10-11 22:13:08.871: W/AudioService(286): Soundpool could not load file: /system/media/audio/ui/KeypressStandard.ogg
  10-11 22:13:08.911: E/SoundPool(286): error loading /system/media/audio/ui/KeypressSpacebar.ogg
  10-11 22:13:08.911: W/AudioService(286): Soundpool could not load file: /system/media/audio/ui/KeypressSpacebar.ogg
  10-11 22:13:08.931: E/SoundPool(286): error loading /system/media/audio/ui/KeypressDelete.ogg
  10-11 22:13:08.971: W/AudioService(286): Soundpool could not load file: /system/media/audio/ui/KeypressDelete.ogg
  10-11 22:13:08.971: E/SoundPool(286): error loading /system/media/audio/ui/KeypressReturn.ogg
  10-11 22:13:08.990: W/AudioService(286): Soundpool could not load file: /system/media/audio/ui/KeypressReturn.ogg
  10-11 22:13:09.001: W/AudioService(286): onLoadSoundEffects(), Error -1 while loading samples
  10-11 22:13:09.052: I/Process(859): Sending signal. PID: 859 SIG: 9
  10-11 22:13:09.521: I/ActivityManager(286): Process net.androidbootcamp.zipcarrentalapp (pid 859) has died.
  10-11 22:13:10.610: I/Choreographer(839): Skipped 603 frames!  The application may be doing too much work on its main thread.
  </pre>
  
  <pre><code>package net.androidbootcamp.zipcarrentalapp;
  
  import android.support.v7.app.ActionBarActivity;
  import android.os.Bundle;
  import android.view.Menu;
  import android.view.MenuItem;
  import android.widget.Button;
  import android.widget.RadioButton;
  import android.widget.RadioGroup;
  import android.widget.TextView;
  
  
  public class MainActivity extends ActionBarActivity 
  {
      private RadioGroup radioGroupId;
      private RadioButton radioButton;
      private Button button;
          final RadioButton Compact = (RadioButton)findViewById(R.id.radCompact);
          final RadioButton MidSize = (RadioButton)findViewById(R.id.radMidSize);
          final RadioButton Luxury = (RadioButton)findViewById(R.id.radLuxury);
          final TextView txtResult = (TextView) findViewById(R.id.txtResult);
          double radCompact = 59.99;
          double radMidSize = 65.99;
          double radLuxury = 89.99;
          int days = 0;
          double computeValue;
  
          public void operation(int days)
          {
              if(days &lt;= 10)
              {
                  if(Compact.isChecked())
                  {
                      computeValue = radCompact * days;
                      txtResult.setText("Cost:" + Double.toString(computeValue));
  
                  }
  
                  else if (MidSize.isChecked())
                  {
                      computeValue = radMidSize * days;
                      txtResult.setText("Cost: " + Double.toString(computeValue));
                  }
                  else if (Luxury.isChecked())
                 {
                      computeValue = radLuxury * days;
                      txtResult.setText("Cost:" + Double.toString(computeValue));
                  }
              }
          }
  
      @Override
      protected void onCreate(Bundle savedInstanceState)
      {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
      }
  
  
      @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);
      }
  }
  </code></pre>