I tried to run this simple code on emulator but it gives error right at the beginning. Helps appreciated. It gives a fatal error in logcat. It says "application has stopped". I get these errors: 
"03-22 17:43:53.315: E/AndroidRuntime(1283): Process: com.example.deneme, PID: 1283"
"03-22 17:43:53.315: E/AndroidRuntime(1283): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.deneme/com.example.deneme.MainActivity}: java.lang.NullPointerException"

```
package com.example.deneme;
import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.os.Bundle;
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.EditText;
import android.os.Build;

public class MainActivity extends Activity {

 final Button button = (Button) findViewById(R.id.button1);
 EditText txt = (EditText) findViewById (R.id.editText1);

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

        button.setOnClickListener(new View.OnClickListener() {

   @Override
   public void onClick(View v) {
    // TODO Auto-generated method stub
   txt.setText("murat");
   }
  });
       // if (savedInstanceState == null) {
         //   getFragmentManager().beginTransaction()
           //         .add(R.id.container, new PlaceholderFragment())
             //       .commit();
      //  }
    }

    @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;
        }
    }

}

```