flex按值引用还是按对象引用

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
			   xmlns:s="library://ns.adobe.com/flex/spark" 
			   xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600" creationComplete="application1_creationCompleteHandler(event)">
	<s:layout>
		<s:BasicLayout/>
	</s:layout>
	<fx:Script>
		<!--[CDATA[
			import com.firemax.controls.FireMaxAlert;
			
			import mx.collections.ArrayCollection;
			import mx.events.FlexEvent;
			
			[Bindable]
			public var listDemo:ArrayCollection;
			
			var stringName:String = new String("李四");
			
			protected function application1_creationCompleteHandler(event:FlexEvent):void
			{
				// TODO Auto-generated method stub
				listDemo=new ArrayCollection();
				
				listDemo.addItem(new String("张三"));
				listDemo.addItem(stringName);
				listDemo.addItem(new String("王五"));
				
				
			}

			protected function button1_clickHandler(event:MouseEvent):void
			{
				// TODO Auto-generated method stub
				stringName="麻子";
				FireMaxAlert.show(listDemo.getItemAt(1).toString());
				//FireMaxAlert.show(stringName);
			}
		]]-->
	</fx:Script>
	<fx:Declarations>
		<!-- 将非可视元素（例如服务、值对象）放在此处 -->
	</fx:Declarations>
	<mx:DataGrid x="46" y="33" width="403" height="212" dataProvider="{listDemo}">
		<mx:columns>
			<mx:DataGridColumn headerText="列 1" dataField="col1"/>
		</mx:columns>
	</mx:DataGrid>
	<s:Button x="46" y="275" label="按钮" click="button1_clickHandler(event)"/>
</s:Application>