bundles/org.eclipse.equinox.p2.core/src/org/eclipse/equinox/internal/provisional/p2/query/MatchQuery.java

/******************************************************************************* 
* Copyright (c) 2009 EclipseSource and others. All rights reserved. This
* program and the accompanying materials are made available under the terms of
* the Eclipse Public License v1.0 which accompanies this distribution, and is
* available at http://www.eclipse.org/legal/epl-v10.html
*
* Contributors:
*   EclipseSource - initial API and implementation
*   IBM Corporation - ongoing development
******************************************************************************/
package org.eclipse.equinox.internal.provisional.p2.query;

import java.util.Iterator;

/**
 * This class represents the superclass of most of p2's queries.  Every element
 * in the query can be evaluated by calling isMatch on it. If {@link #isMatch(Object)} returns true, 
 * then the element WILL be included in the query result.  If {@link #isMatch(Object)} returns false, then 
 * the element WILL NOT be included in the query result.
 * <p>
 * This class may be subclassed by clients. Subclasses should specify the type
 * of object they support querying on. Subclasses are also encouraged to clearly
 * specify their match algorithm, and expose the parameters involved in the match
 * computation, to allow {@link IQueryable} implementations to optimize their
 * execution of the query. 
 */
public abstract class MatchQuery implements IMatchQuery {

	/**
	 * Returns whether the given object satisfies the parameters of this query.
	 * 
	 * @param candidate The object to perform the query against
	 * @return <code>true</code> if the unit satisfies the parameters
	 * of this query, and <code>false</code> otherwise
	 */
	public abstract boolean isMatch(Object candidate);

	/**
	 * Performs this query on the given iterator, passing all objects in the iterator 
	 * that match the criteria of this query to the given result.
	 */
	public final Collector perform(Iterator iterator, Collector result) {
		prepareToPerform();
		try {
			while (iterator.hasNext()) {
				Object candidate = iterator.next();
				if (isMatch(candidate))
					if (!result.accept(candidate))
						break;
			}
		} finally {
			performComplete();
		}
		return result;
	}

	/**
	 * Execute any pre-processing that must be done before this query is performed against
	 * a particular iterator.  This method may be used by subclasses to do any calculations,
	 * caching, or other preparation for the query.
	 * <p>
	 * This method is internal to the framework.  Subclasses may override this method, but
	 * should not call this method.
	 */
	protected void prepareToPerform() {
		// nothing to do by default
	}

	/**
	 * Execute any post-processing that must be done after this query has been performed against
	 * a particular iterator.  This method may be used by subclasses to clear caches or any other
	 * cleanup that should occur after a query.  
	 * <p>
	 * This method will be called even if the query does not complete successfully.
	 * <p>
	 * This method is internal to the framework.  Subclasses may override this method, but
	 * should not call this method.
	 */
	protected void performComplete() {
		// nothing to do by default
	}
}