问下各位，为什么小弟我的AlertDialog.Builder没有dismiss和cancel

问下各位，为什么我的AlertDialog.Builder没有dismiss和cancel
AlertDialog.Builder builder1 = new AlertDialog.Builder(MainActivity.this);
builder1.setMessage("aaaaa").show();
builder1.cancel();

会提示错误
cannot find symbol
  symbol: method cancel()
  location: variable builder1 of type android.app.AlertDialog.Builder
----
什么回事捏？

------解决方案--------------------
AlertDialog dialog = builder1.create();
dialog.dismiss();