Binding expressions and collections

Question

<quote>
In addition to being able to read the initial value of a field from an object, value binding expressions can also be used to navigate maps, lists, or arrays. Here are some examples:
#{foo[bar]}
#{foo["bar"]}
#{foo[3]}
#[foo[3].bar}
#{foo.bar[3]}
</quote>
Could anyone explain how does it work ? Is foo is always bean ?
Isn't #{foo["bar"]} identical to #{foo.bar}
Also in "Core JSF" they say #{foo.bar[3]} doesn't work if bar indexed property of a bean.
<anotherquote>
The syntax of a method binding expression has two primary formats:
#{expr-a.value-b}
#{expr-a[value-b]}
The first format is a classic DOM-style object tree structure. The second format can be used to call a method pointed to from an object array, Map, or List.
</anotherquote>
The same question how to use the second form.

Answer 1

An expression is a little piece of softwaremuch like a scriptthat evaluates to a single value for a single layer property at a specific point in time. Whereas scripts tell an application to _do_ something, an expression says that a property _is_ something.
With expressions, you can create relationships between layer properties and use the keyframes of one property to dynamically animate other layers. For example, you can use the pick whip to link path properties, so a mask can take its path from a brush stroke or a shape layer object.
The expression language is based on the standard JavaScript language, but you do not need to know JavaScript to use expressions. You can create expressions by using the pick whip or by copying simple examples and modifying them to suit your needs.
Be sure to read the
"Expressions" section of After Effects Help on the Web.
Dan Ebberts has an excellent collection of example expressions and scripts and tutorials for learning how to work with expressions and scripts on his website: http://www.adobe.com/go/learn_ae_motionscripthome.
The AE Enhancers forum provides many examples and much information about expressions, as well as scripts and animation presets: http://www.adobe.com/go/learn_ae_aeenhancershome.
JJ Gifford provides several example projects on his website that demonstrate how to use expressions: http://www.adobe.com/go/learn_ae_jjgiffordexpressions.