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   package cz.cuni.amis.utils.collections;
   
   import java.util.Collection;
   
  /**
   * Collection listener with only one method changed() called on each change.
   * @author ik
   */
  public abstract class SimpleListener<T> implements CollectionEventListener<T> {
  
      @Override
      public void preAddEvent(Collection<T> toBeAdded, Collection<T> whereToAdd) {
      }
  
      @Override
      public void postAddEvent(Collection<T> alreadyAdded, Collection<T> whereWereAdded) {
          changed(whereWereAdded, alreadyAdded, null);
      }
  
      @Override
      public void preRemoveEvent(Collection<T> toBeRemoved, Collection<T> whereToRemove) {
      }
  
      @Override
      public void postRemoveEvent(Collection<T> alreadyRemoved, Collection<T> whereWereRemoved) {
          changed(whereWereRemoved, null, alreadyRemoved);
      }
  
      /**
       * Called when a change occures on the collection.
       * @param collection The changed collection.
       */
      protected abstract void changed(Collection<T> collection, Collection<T> added, Collection<T> removed);
  
  }