对话框的标题和内容都可以显示出来，为什么定义的数组列表显示不出来呢？

Question

public class SingleActivity extends Activity { Button bt; String aa[]={"1","2","3","4","5",}; protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_single); bt=(Button) findViewById(R.id.button1); final AlertDialog.Builder B = new AlertDialog.Builder(SingleActivity.this); B.setTitle("单选对话框"); B.setMessage("请选择"); B.setSingleChoiceItems(aa, 0, new DialogInterface.OnClickListener() { @Override public void onClick(DialogInterface arg0, int arg1) { // TODO Auto-generated method stub String str=aa[arg1]; Toast.makeText(SingleActivity.this, str, Toast.LENGTH_LONG); } }); bt.setOnClickListener(new OnClickListener() { public void onClick(View arg0) { // TODO Auto-generated method stub AlertDialog ad=B.create(); ad.show(); } }); }}