问当用户尝试返回时，Android对话框不会完全消失

EN

public class DialogActivity extends Activity {
AlertDialog alertDialog;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Intent intent = getIntent();
    AlertDialog alertDialog = new AlertDialog.Builder(this).create();
    alertDialog.setTitle("Title");
    alertDialog.setMessage("Body");
    alertDialog.setIcon(R.drawable.ic_launcher);
    aleratDialog.setButton("Ok", new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog, int which) { 
            DialogActivity.this.finish();
        }
    });

    alertDialog.show();
}

@Override
protected void onPause() {
    if(alertDialog!=null) {alertDialog.dismiss();}
    super.onPause();

}

@Override
protected void onStop() {
    if(alertDialog!=null) {alertDialog.dismiss();}
    super.onStop();

}
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if ((keyCode == KeyEvent.KEYCODE_BACK)) {
        finish();
    }
    return super.onKeyDown(keyCode, event);
}


}

<activity
        android:name=".DialogActivity"
        android:launchMode="singleInstance"
        android:noHistory="true"
        android:label="@string/app_name"
        android:configChanges="orientation"
        android:theme="@android:style/Theme.Holo.Dialog" />