понедельник, 18 января 2010 г.

Getting changes from collection

Good morning :)

Usually to update some entities in database I use service that must be calculate changes from new and old object and update changes in database(using DAO, for example). And the common task is update collections. I wrote simple class that calculate changes from old and new collections and create three collections: to delete, to update and to create. You can see class below and some description for it

package com.blogspot.jajatips.utils;

import org.apache.commons.collections.CollectionUtils;
import org.apache.commons.collections.Predicate;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import java.util.*;

public class ChangesCollections<T> {
 private static class CollectionFactory<T> {
  private static final Log log = LogFactory.getLog(CollectionFactory.class);
  private Class<? extends Collection> classCollection = ArrayList.class;

  private CollectionFactory(Class<? extends Collection> classCollection) {
   this.classCollection = classCollection;
  }

  @SuppressWarnings("unchecked")
  public Collection<T> getNewCollection() {
   try {
    return classCollection.newInstance();
   } catch (Exception e) {
    log.warn("Can not create collection for class [" + classCollection.getName() + "], create java.util.ArrayList");
    return new ArrayList<T>();
   }
  }
 }

 private Collection<T> deleteCollection;
 private Collection<T> addCollection;
 private Collection<T> updateCollection;

 public ChangesCollections(Collection<T> oldCollection, Collection<T> newCollection) {
  CollectionFactory<T> factory = new CollectionFactory<T>(oldCollection.getClass());

  this.deleteCollection = factory.getNewCollection();
  this.addCollection = factory.getNewCollection();
  this.updateCollection = factory.getNewCollection();

  addCollection.addAll(newCollection);
  deleteCollection.addAll(oldCollection);
  Iterator<T> addIt = addCollection.iterator();
  while (addIt.hasNext()) {
   T addObject = addIt.next();
   if (contains(oldCollection, addObject)) {
    updateCollection.add(addObject);
    deleteCollection.remove(addObject);
    addIt.remove();
   }
  }
 }

 public Collection<T> getDeleteCollection() {
  return deleteCollection;
 }

 public Collection<T> getAddCollection() {
  return addCollection;
 }

 public Collection<T> getUpdateCollection() {
  return updateCollection;
 }

 private boolean contains(Collection<T> collection, T object) {
  if (object.getClass().isAssignableFrom(Predicate.class)) {
   return CollectionUtils.exists(collection, (Predicate) object);
  }

  return collection.contains(object);
 }
}

First of all, to build this code you need next libraries: commons-collections and commons-logging. Also, I like generics and use it in all my code, because I like compile errors more than runtime errors :).
Now, about the code. To create collections, I use inner class CollectionFactory, that try to create class of collection same as original collection, if it can not do this, it create standard java.util.List. As you can see, code is simple. All that it done is iterate over collections and put elements in other three collections.

I hope that this class helps you in your projects :)

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