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*** THIS VERSION UPLOADED FOR USE WITH CEDAR-COMMON, TO AVOID DEPENDENCIES ON GOOGLE CODE-BASED MAVEN REPOSITORIES. *** The simplest convenient interface to the Google App Engine datastore

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package com.googlecode.objectify.impl.conv;


/** 
 * Interface for objects which know how to convert from POJO field types to
 * whatever is necessary for native storage in the datastore.  The datastore
 * Entity has significant limits on what can be saved with setProperty()
 * and inadvertently transmutes some values (eg Integer to Long).  The converter
 * knows how to make it all work.
 * 
 * Converters are assembled in a chain and each one is given a crack at
 * the value.  This is because some converters cover broad ranges of subclasses
 * (eg, Enum) and a simple hashmap lookup is inadequate.  The first converter
 * that returns a non-null value "wins".
 * 
 * THIS API IS EXPERIMENTAL.  It may change significantly in minor point releases.
 */
public interface Converter
{
	/** 
	 * Convert the value into an object suitable for storage in the datastore.
	 * The first thing converters should do is test whether they are appropriate
	 * for the input data; if not, return null.
	 * 
	 * @param value will never be null
	 * @return null to indicate that this converter does nothing with the value
	 */
	Object forDatastore(Object value, ConverterSaveContext ctx);

	/**
	 * Convert the value into an object suitable for setting on a field with
	 * the specified type.
	 * The first thing converters should do is test whether they are appropriate
	 * for the input data; if not, return null.
	 * 
	 * @param value will never be null
	 * @param fieldType is the type that the value should be converted to
	 * @param onPojo is the actual pojo object that this value will eventually be set upon
	 * @return null to indicate that this converter does nothing with the value
	 */
	Object forPojo(Object value, Class fieldType, ConverterLoadContext ctx, Object onPojo);
}