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The AspectJ weaver applies aspects to Java classes. It can be used as a Java agent in order to apply load-time weaving (LTW) during class-loading and also contains the AspectJ runtime classes.
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/* *******************************************************************
 * Copyright (c) 1999-2001 Xerox Corporation,
 *               2002 Palo Alto Research Center, Incorporated (PARC).
 * All rights reserved.
 * This program and the accompanying materials are made available
 * under the terms of the Eclipse Public License v 2.0
 * which accompanies this distribution and is available at
 * https://www.eclipse.org/org/documents/epl-2.0/EPL-2.0.txt
 *
 * Contributors:
 *     Xerox/PARC     initial implementation
 * ******************************************************************/

package org.aspectj.util;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

/**
 * This class implements a partial order
 *
 * It includes routines for doing a topo-sort
 */

public class PartialOrder {

	/**
	 * All classes that want to be part of a partial order must implement PartialOrder.PartialComparable.
	 */
	public interface PartialComparable {
		/**
		 * @return 
		 *         +1 if this is greater than other
		 *         -1 if this is less than other
		 *         0 if this is not comparable to other
		 *         
		 *
		 *          Note: returning 0 from this method doesn't mean the same thing as returning 0 from
		 *         java.util.Comparable.compareTo()
		 */
		int compareTo(Object other);

		/**
		 * This method can provide a deterministic ordering for elements that are strictly not comparable. If you have no need for
		 * this, this method can just return 0 whenever called.
		 */
		int fallbackCompareTo(Object other);
	}

	private static class SortObject {
		T object;
		List> smallerObjects = new LinkedList<>();
		List> biggerObjects = new LinkedList<>();

		public SortObject(T o) {
			object = o;
		}

		boolean hasNoSmallerObjects() {
			return smallerObjects.size() == 0;
		}

		boolean removeSmallerObject(SortObject o) {
			smallerObjects.remove(o);
			return hasNoSmallerObjects();
		}

		void addDirectedLinks(SortObject other) {
			int cmp = object.compareTo(other.object);
			if (cmp == 0) {
				return;
			}
			if (cmp > 0) {
				this.smallerObjects.add(other);
				other.biggerObjects.add(this);
			} else {
				this.biggerObjects.add(other);
				other.smallerObjects.add(this);
			}
		}

		public String toString() {
			return object.toString(); // +smallerObjects+biggerObjects;
		}
	}

	private static  void addNewPartialComparable(List> graph, T o) {
		SortObject so = new SortObject<>(o);
		for (SortObject other : graph) {
			so.addDirectedLinks(other);
		}
		graph.add(so);
	}

	private static  void removeFromGraph(List> graph, SortObject o) {
		for (Iterator> i = graph.iterator(); i.hasNext();) {
			SortObject other = i.next();

			if (o == other) {
				i.remove();
			}
			// ??? could use this to build up a new queue of objects with no
			// ??? smaller ones
			other.removeSmallerObject(o);
		}
	}

	/**
	 * @param objects must all implement PartialComparable
	 *
	 * @return the same members as objects, but sorted according to their partial order. returns null if the objects are cyclical
	 *
	 */
	public static  List sort(List objects) {
		// lists of size 0 or 1 don't need any sorting
		if (objects.size() < 2) {
			return objects;
		}

		// ??? we might want to optimize a few other cases of small size

		// ??? I don't like creating this data structure, but it does give good
		// ??? separation of concerns.
		List> sortList = new LinkedList<>();
		for (T object : objects) {
			addNewPartialComparable(sortList, object);
		}

		// System.out.println(sortList);

		// now we have built our directed graph
		// use a simple sort algorithm from here
		// can increase efficiency later
		// List ret = new ArrayList(objects.size());
		final int N = objects.size();
		for (int index = 0; index < N; index++) {
			// System.out.println(sortList);
			// System.out.println("-->" + ret);

			SortObject leastWithNoSmallers = null;

			for (SortObject so: sortList) {
				if (so.hasNoSmallerObjects()) {
					if (leastWithNoSmallers == null || so.object.fallbackCompareTo(leastWithNoSmallers.object) < 0) {
						leastWithNoSmallers = so;
					}
				}
			}

			if (leastWithNoSmallers == null) {
				return null;
			}

			removeFromGraph(sortList, leastWithNoSmallers);
			objects.set(index, leastWithNoSmallers.object);
		}

		return objects;
	}

	/***********************************************************************************
	 * /* a minimal testing harness
	 ***********************************************************************************/
	static class Token implements PartialComparable {
		private String s;

		Token(String s) {
			this.s = s;
		}

		public int compareTo(Object other) {
			Token t = (Token) other;

			int cmp = s.charAt(0) - t.s.charAt(0);
			if (cmp == 1) {
				return 1;
			}
			if (cmp == -1) {
				return -1;
			}
			return 0;
		}

		public int fallbackCompareTo(Object other) {
			return -s.compareTo(((Token) other).s);
		}

		public String toString() {
			return s;
		}
	}

	public static void main(String[] args) {
		List l = new ArrayList<>();
		l.add(new Token("a1"));
		l.add(new Token("c2"));
		l.add(new Token("b3"));
		l.add(new Token("f4"));
		l.add(new Token("e5"));
		l.add(new Token("d6"));
		l.add(new Token("c7"));
		l.add(new Token("b8"));

		l.add(new Token("z"));
		l.add(new Token("x"));

		l.add(new Token("f9"));
		l.add(new Token("e10"));
		l.add(new Token("a11"));
		l.add(new Token("d12"));
		l.add(new Token("b13"));
		l.add(new Token("c14"));

		System.out.println(l);

		sort(l);

		System.out.println(l);
	}
}