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Resource merging for Apache Wicket (http://wicket.apache.org), see http://talk-on-tech.blogspot.com/2008/08/wicket-interface-speed-up-merging.html

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/**
 * Copyright 2010 Molindo GmbH
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.wicketstuff.mergedresources.versioning;

import org.apache.wicket.IClusterable;

public abstract class AbstractResourceVersion implements IClusterable {
	private static final long serialVersionUID = 1L;

	public static AbstractResourceVersion NO_VERSION = new AbstractResourceVersion() {

		private static final long serialVersionUID = 1L;

		public String getVersion() {
			return "";
		}

		public boolean isValid() {
			return false;
		}

		public int compareValid(AbstractResourceVersion o) {
			// shouldn't ever be called
			return -1;
		}

	};

	public abstract boolean isValid();

	public abstract String getVersion();

	/**
	 * @param o
	 *            the object to be compared.
	 * @return a negative integer, zero, or a positive integer as this object is
	 *         less than, equal to, or greater than the specified object.
	 * @throws IncompatibleVersionsException
	 */
	public final int compareTo(AbstractResourceVersion o) throws IncompatibleVersionsException {
		if (!o.isValid()) {
			return isValid() ? 1 : 0;
		} else {
			return isValid() ? compareValid(o) : -1;
		}

	}

	protected abstract int compareValid(AbstractResourceVersion o) throws IncompatibleVersionsException;

	public static final class IncompatibleVersionsException extends Exception {

		public IncompatibleVersionsException(AbstractResourceVersion v1, AbstractResourceVersion v2) {
			super(v1 + " and " + v2 + " are incompatible");
		}

		private static final long serialVersionUID = 1L;

	}
}