引用android原生的AlertDialog

var AlertDialog = plus.android.importClass('android.app.AlertDialog');  
                            var adb = new AlertDialog.Builder(plus.android.runtimeMainActivity());  
                            var listener = plus.android.implements('android.content.DialogInterface.OnClickListener',{  
                                'onClick':function(dialog,which)  
                                {  
                                    fruitPlus.downloadWgt();  
                                }  
                            });  
                            adb.setTitle("是否更新？");  
                            adb.setPositiveButton("确定",listener);  
                            adb.setNegativeButton("取消",null);  
                            adb.setCancelable(false);  
                            adb.show();

这段代码显示的AlertDialog怎么只有取消按钮