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<p>Provides interfaces for the data layer which contains classes
for typed data values, data collections, and handlers. A 
{@link com.itmill.toolkit.data.Property Property} is a simple typed data
value; an {@link com.itmill.toolkit.data.Item Item} is a collection of
Properties, each corresponding to a unique identifier; a
{@link com.itmill.toolkit.data.Container Container} is a collection of
identified Items with special constraints; a
{@link com.itmill.toolkit.data.Buffered Buffered} class is able to track
its changes and to commit or discard them later.</p>

<h2>Package Specification</h2>

<p>The package contains a three-tiered structure for typed data objects and
collections of them:</p>

<center>
<p><a href=Property.html target="classFrame"><img src=doc-files/Property.gif></a></p>
<p><a href=Item.html target="classFrame"><img src=doc-files/Item.gif></a></p>
<p><a href=Container.html target="classFrame"><img src=doc-files/Container_simple.gif></a></p>
</center>

<ul>
	<li>The simplest of these is the
	{@link com.itmill.toolkit.data.Property Property} which represents a
	single typed data value. A Property may be read-only in which
	case attempts to modify its contents will throw an exception.	 

	<li>The second level of the data layer is represented by the
	{@link com.itmill.toolkit.data.Item Item} which embodies a set of
	<i>Properties</i>. Each Property in an Item corresponds to a locally
	unique(that is, inside the Item) identifier.
	
	<li>The third level is called the
	{@link com.itmill.toolkit.data.Container Container} which can be
	visualized as a set of Items, each corresponding to a locally unique
	identifier. Note that the Container imposes a few restrictions on the
	data stored in it, see further documentation in the
	<a href=Container.html target="classFrame">class specification</a>.
</ul>

<p>In addition to these interfaces the package contains the 
{@link com.itmill.toolkit.data.Buffered Buffered} interface, which defines
the methods to make an object buffered, that is, track the changes to an
object and allow committing or discarding them at a later time.</p>

<p>Provides interfaces for the validation framework. The framework
defines two interfaces; one for classes that need to support external
validation, and another one for the validators themselves.</p>

<h2>Validation</h2>

<p>The most important method defined by the
{@link com.itmill.toolkit.data.Validatable Validatable} interface is
{@link com.itmill.toolkit.data.Validatable#isValid() isValid()}. It
asks all registered validators to verify if the object's value is valid
or not. Note that it depends on the validators registered for a object which
values are valid and which are not. For example, a <code>null</code> value
can be valid value for one validator but invalid for another.<p>

<p>In addition to <code>isValid()</code>, <code>Validatable</code> defines
methods to add, remove and list validators of a validatable object.</p>

<p>{@link com.itmill.toolkit.data.Validator Validator} defines the
interface for an external validator.  These validators may be added to
any <code>Validatable</code> object, and their task is to check, when
requested, that the object which they are attached to contains a valid
value. The actual validation logic is hidden in the 
{@link com.itmill.toolkit.data.Validator#validate(Object) validate(Object)}
method.</p>

<p>In addition to <code>check(Object)</code>, <code>Validator</code> defines
the <code>InvalidValueException</code> which is used to signal that a
checked value is invalid, and the
{@link com.itmill.toolkit.data.Validator.Suggestive Suggestive}
subinterface which includes functionality to suggest a valid value for
the validated object.</p><!-- Put @see and @since tags down here. -->

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