关于提示eclipse上,“XX出错，程序已停止运行”的原因

详细说下，先在布局里添加一个button和textview控件，然后在mainactivity里实现点击按钮设置textview的显示内容，个人感觉思路很简单啊，代码也没报错，就是一运行就终止，实在不知其所以然了。以下是代码，注意中间一行我给注释了，不注释就会运行停止。

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.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.os.Build;

public class MainActivity extends Activity {

private Button button;
private TextView textView;

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

    button = (Button)findViewById(R.id.button);
    textView = (TextView)findViewById(R.id.textview);

  //  button.setOnClickListener(new Buttonlistener());
    if (savedInstanceState == null) {
        getFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
}

public class Buttonlistener implements OnClickListener{

@Override
public void onClick(View arg0) {
    textView.setText("NIHAO ");

}
}
@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;
    }
}
}