I know that class names should begin with an uppercase first letter and are usually nouns. start with lowercase letters is really bad. But that is only the guidelines apply to naming classes, isn’t it?
BTW, I read the following post about class name:
http://www.adobe.com/devnet/flash/articles/as_bestpractices_02.html
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The following guidelines apply to naming classes:

* Begin a class name with an uppercase letter.
* Write class names in mixed case when it’s a compound or concatenated word.
* Begin with an uppercase letter for a compound or concatenated word. A good example is NewMember.
* Class names are usually nouns or qualified nouns. A qualifier describes the noun or phrase. For example, instead of “member,” you might qualify the noun by using NewMember or OldMember.
* Clear names are more important than short names.
* Don’t use acronyms and abbreviations. The exception to this rule is if acronyms or abbreviations represent the standard way to use a term (such as HTML or CFM). For commonly used acronyms, use mixed cases such as NewHtmlParser instead of NewHTMLParser for improved readability.
* Use meaningful and simple names that are descriptive of the class contents. To avoid being vague or misleading, use generic names.
* Sometimes a class name is a compound word. A qualifier might describe the noun or phrase. For example, instead of “member,” you might qualify the noun using NewMember or OldMember.
* Do not pluralize the words you use in the class name (such as Witches or BaldPirates). In most cases, it is better to leave the words as qualified nouns instead. A qualifier describes the noun or phrase. For example, instead of “cat” or “buckaneer,” you might qualify the noun by using BlackCat or OldBuckaneer.
* Don’t use a class name in the properties of that class because it causes redundancy. For example, it does not make sense to have Cat.catWhiskers. Instead, Cat.whiskers is much better.
* Don’t use nouns that also might be interpreted as verbs. For example, Running, or Gardening. Using these nouns might lead to confusion with methods, states, or other application activities.
* Use unique class names for each class in a single application.
* Do not name classes so that they conflict with the names of built-in classes in Flash.
* Try to communicate the relationship that a class has within a hierarchy. This helps display a class’s relationship within an application. For example, you might have the Widget interface, and the implementation of Widget might be PlasticWidget, SteelWidget, and SmallWidget.
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anyway, thanks for your advice and I will fix it from next post. ( although flex builder allow me type lowercase letters as my class name. )