Android :: Process Dialog For Long Operation - Thread Object

In the code showed as below I create a process dialog for doing some long
operation (in this case waiting 4 seconds). It runs as it should. And shows the "Starting" and "Done" message but when it finishes and I click Button1 again it terminates.

public class main extends Activity { public ProgressDialog dialog = null;
final Handler handler = new Handler(){ public void handleMessage(Message msg){
dialog.dismiss(); TextView tv2 = (TextView) findViewById(R.id.TextView02);
tv2.setText("Done"); } };
Thread runlongjob = new Thread(){ public void run(){
SystemClock.sleep(4000); handler.sendEmptyMessage(0);
} };
/** Called when the activity is first created. */
@Override public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState); setContentView(R.layout.main);
} public void clickhandler(View v){ switch(v.getId()){
case (R.id.Button01): TextView tv1 = (TextView) findViewById(R.id.TextView01);
tv1.setText("Starting...");
dialog = ProgressDialog.show(main.this, "Please wait", "Doing Job...",
true); runlongjob.start(); break; } }