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  package org.apache.cayenne.map;
  
  import java.io.Serializable;
  
  import org.apache.cayenne.CayenneRuntimeException;
  import org.apache.cayenne.util.CayenneMapEntry;
  import org.apache.cayenne.util.ToStringBuilder;
  import org.apache.cayenne.util.XMLSerializable;
  
  /**
   * Defines a relationship between two entities. In a DataMap graph relationships represent
   * "arcs" connecting entity "nodes". Relationships are directional, i.e. they have a
   * notion of source and target entity. This makes DataMap a "digraph".
   */
  public abstract class Relationship<E extends Entity<E, A, R>, A extends Attribute<E, A, R>, R extends Relationship<E, A, R>>
          implements CayenneMapEntry, XMLSerializable, Serializable {
  
      protected String name;
      protected E sourceEntity;
  
      protected String targetEntityName;
      protected boolean toMany;
  
      /**
       * A flag that specifies whether a Relationship was mapped by the user or added
       * dynamically by Cayenne runtime.
       * 
       * @since 3.0
       */
      protected boolean runtime;
  
      /**
       * Creates an unnamed relationship.
       */
      public Relationship() {
      }
  
      /**
       * Creates a named relationship.
       */
      public Relationship(String name) {
          setName(name);
      }
  
      public String getName() {
          return name;
      }
  
      public void setName(String name) {
          this.name = name;
      }
  
      /**
       * Returns relationship source entity.
       */
      public E getSourceEntity() {
          return sourceEntity;
      }
  
      /**
       * Sets relationship source entity.
       */
      public void setSourceEntity(E sourceEntity) {
          this.sourceEntity = sourceEntity;
      }
  
      /**
       * Returns a target entity of the relationship.
       */
      public abstract E getTargetEntity();
  
      /**
       * Sets relationship target entity. Internally calls <code>setTargetEntityName</code>.
       */
      public void setTargetEntityName(Entity<E, A, R> targetEntity) {
          if (targetEntity != null) {
              setTargetEntityName(targetEntity.getName());
          } else {
              setTargetEntityName((String) null);
          }
      }
  
      /**
       * Returns the name of a target entity.
       */
      public String getTargetEntityName() {
          return targetEntityName;
      }
  
      /**
       * Sets the name of relationship target entity.
       */
      public void setTargetEntityName(String targetEntityName) {
          this.targetEntityName = targetEntityName;
      }
  
      /**
       * Returns a boolean value that determines relationship multiplicity. This defines
       * semantics of the connection between two nodes described by the source and target
       * entities. E.g. to-many relationship between two Persistent object classes means
       * that a source object would have a collection of target objects. This is a read-only
       * property.
       */
      public boolean isToMany() {
          return toMany;
      }
  
      public Object getParent() {
          return getSourceEntity();
      }
  
      @SuppressWarnings("unchecked")
      public void setParent(Object parent) {
          if (parent != null && !(parent instanceof Entity)) {
              throw new IllegalArgumentException("Expected null or Entity, got: " + parent);
          }
  
          setSourceEntity((E) parent);
      }
  
      /**
       * Returns guaranteed non-null MappingNamespace of this relationship. If it happens to
       * be null, and exception is thrown. This method is intended for internal use by
       * Relationship class.
       */
      final MappingNamespace getNonNullNamespace() {
          E entity = getSourceEntity();
  
          if (entity == null) {
              throw new CayenneRuntimeException("Relationship '%s' has no parent Entity.", getName());
          }
  
          return entity.getNonNullNamespace();
      }
  
      /**
       * Overrides Object.toString() to return informative description.
       */
      @Override
      public String toString() {
          return new ToStringBuilder(this)
                  .append("name", getName())
                  .append("toMany", isToMany()).toString();
      }
  
      /**
       * @since 3.0
       */
      public boolean isRuntime() {
          return runtime;
      }
  
      /**
       * @since 3.0
       */
      public void setRuntime(boolean synthetic) {
          this.runtime = synthetic;
      }
      
      /**
       * Returns a "complimentary" relationship going in the opposite direction. Returns
       * null if no such relationship is found.
       * @since 3.1
       */
      public abstract Relationship<E, A, R> getReverseRelationship();
      
      /**
       * Returns if relationship is mandatory
       * @since 3.1
       */
      public abstract boolean isMandatory();
  }
  