import java.util.ArrayList;import java.util.HashMap;import java.util.ListIterator;/** * <pre>    * Topology class keeps track of connections between nodes.  * This is a Singleton object * </pre>    */public class Topology {        private ArrayList edges;    private HashMap failedNodes;    private boolean allToAll;    private static Topology topology = null;    /**     * Static method for getting an instance of Topology     * @return An instance of Topology     */    public static Topology GetInstance() {	if(Topology.topology == null) {	    Topology.topology = new Topology(false);	    	}    	return Topology.topology;    }    /**     * Static method for getting an instance of Topology     * @return An instance of Topology     */    public static Topology GetInstance(boolean allToAll) {	if(Topology.topology == null) {	    Topology.topology = new Topology(allToAll);	    	}    	return Topology.topology;    }    /**     * Returns the edge between a and b, if one exists and it is live. Returns null otherwise     * @param a Int specifying a node     * @param b Int specifying a node     * @return The live edge between a and b. If one does not exist then null is returned     */    public Edge getLiveEdge(int a, int b) {	Edge e = getEdge(a, b);	if(e == null || !e.isLive() || !this.isNodeAlive(a) || !this.isNodeAlive(b)) {	    return null;	} 	return e;    }    /**     * Returns true if the given node is alive, else return false     * @param node Int specifying node     */    public boolean isNodeAlive(int node) {	return !this.failedNodes.containsKey(new Integer(node));    }    /**     * Creates a new edge or updates an existing one     * @param a Int specifying a node     * @param b Int specifying a node     * @param options Options to set for the given edge     */    public void newEdge(int a, int b, EdgeOptions options) {	Edge e = this.getEdge(a, b);	if (e != null) {	    e.setState(true);	}else {	    e = new Edge(a, b, options);	    edges.add(e);	}    }    /**     * Mark the given edge as failed so that we don't use it     * @param a Int specifying a node     * @param b Int specifying a node     * @return True if edge was succesfully failed. False if no edge exists between a and b     */    public boolean failEdge(int a, int b) {	return changeEdge(a, b, false);    }    /**     * Restart the given edge so that we can use it again     * @param a Int specifying a node     * @param b Int specifying a node     * @return True if edge was succesfully restarted. False if no edge exists between a and b     */    public boolean restartEdge(int a, int b) {	return changeEdge(a, b, true);    }        /**     * Mark the given node as failed, so that we don't use any edge that goes through it     * @param a Int specifying a node     */    public void failNode(int a) {	this.failedNodes.put(new Integer(a), null);    }    /**     * Mark the given node as ok, so that we can use it again     * @param a Int specifying a node     */    public void restartNode(int a) {	this.failedNodes.remove(new Integer(a));    }    //********** Private Functions **********    private Topology(boolean allToAll) {	this.edges = new ArrayList();	this.failedNodes = new HashMap();	this.allToAll = allToAll;    }    private Edge getEdge(int a, int b) {	ListIterator iter = this.edges.listIterator();	Edge e;	while(iter.hasNext()) {	    e = (Edge)iter.next();	    if (e.isEdge(a, b)) {		return e;	    }	}	if(this.allToAll) {	    // if no edge exists create one	    e = new Edge(a, b, new EdgeOptions());	    return e;	}	return null;    }        private boolean changeEdge(int a, int b, boolean state) {	Edge e = this.getEdge(a, b);	if(e != null) {	    e.setState(state);	    return true;	}		System.err.println("No edge exists between " + String.valueOf(a) + " and " + String.valueOf(b));		return false;    }}