cayenne-server/src/main/java/org/apache/cayenne/ValueHolder.java

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package org.apache.cayenne;

import java.io.Serializable;

/**
 * Provides a level of indirection for property value access, most often used for deferred
 * faulting of to-one relationships. A ValueHolder abstracts how a property value is
 * obtained (fetched from DB, etc.), thus simplifying design of an object that uses it.
 * <p>
 * Here is an example of a bean property implemented using ValueHolder:
 * </p>
 * 
 * <pre>
 * protected ValueHolder someProperty;
 * 
 * public SomeClass getSomeProperty() {
 *     return (SomeClass) somePropertyHolder.getValue(SomeClass.class);
 * }
 * 
 * public void setSomeProperty(SomeClass newValue) {
 *     somePropertyHolder.setValue(SomeClass.class, newValue);
 * }
 * </pre>
 * 
 * @since 1.2
 */
public interface ValueHolder<E> extends Serializable {

    /**
     * Returns an object stored by this ValueHolder.
     */
    E getValue() throws CayenneRuntimeException;

    /**
     * Retrieves ValueHolder value without triggering fault resolution.
     */
    E getValueDirectly() throws CayenneRuntimeException;

    /**
     * Sets an object stored by this ValueHolder.
     * 
     * @param value a new value of the ValueHolder.
     * @return a previous value saved in the ValueHolder.
     */
    E setValue(E value) throws CayenneRuntimeException;

    /**
     * Sets ValueHolder vaue without triggering fault resolution.
     */
    E setValueDirectly(E value) throws CayenneRuntimeException;

    /**
     * Returns true if the internal value is not yet resolved.
     */
    boolean isFault();

    /**
     * Turns a ValueHolder into a fault.
     */
    void invalidate();
}