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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();
  }
  