vertex.java

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// Vertex base class, to be extended as needed.
// (c) 1998, 2001 duane a. bailey

package structure;

/**
 * A private implementation of a vertex for use in graphs.  A vertex
 * is capable of holding a label and has a flag that can be set  
 * to mark it as visited.   
 * <P>
 * Typical Usage:
 * <P>
 * <pre>
 *     Vertex v = new {@link #Vertex(Object) Vertex(someLabel)};
 *     //...several graph related operations occur
 *     if(!v.{@link #isVisited()}){
 *         Object label = v.label();
 *         v.{@link #visit()};
 *     }
 * </pre>
 * @see GraphListVertex
 * @see GraphMatrixVertex
 *        
 * @version $Id: Vertex.java,v 4.0 2000/12/27 20:57:33 bailey Exp bailey $
 * @author, 2001 duane a. bailey
 */
class Vertex
{
    /**
     * A label associated with vertex.
     */
    protected Object label;	// the user's label
    /**
     * Whether or not a vertex has been visited.
     */
    protected boolean visited;	// this vertex visited

    /**
     * Construct a vertex with an associated label.
     *
     * @post constructs unvisited vertex with label
     * 
     * @param label A label to be associated with vertex.
     */
    public Vertex(Object label)
    {
	Assert.pre(label != null, "Vertex key is non-null");
	this.label = label;
	visited = false;
    }

    /**
     * Fetch the label associated with vertex.
     *
     * @post returns user label associated w/vertex
     * 
     * @return The label associated with vertex.
     */
    public Object label()
    {
	return label;
    }

    /**
     * Test and set the visited flag.
     *
     * @post returns, then marks vertex as being visited
     * 
     * @return The value of the flag before marking
     */
    public boolean visit()
    {
	boolean was = visited;
	visited = true;
	return was;
    }

    /**
     * Determine if the vertex has been visited.
     *
     * @post returns true iff vertex has been visited
     * 
     * @return True iff the vertex has been visited.
     */
    public boolean isVisited()
    {
	return visited;
    }
 
    /**
     * Clears the visited flag.
     *
     * @post marks vertex unvisited
     */
    public void reset()
    {
	visited = false;
    }
    
    /**
     * Returns true iff the labels of two vertices are equal.
     *
     * @post returns true iff vertex labels are equal
     * 
     * @param o Another vertex.
     * @return True iff the vertex labels are equal.
     */
    public boolean equals(Object o)
    {
	Vertex v = (Vertex)o;
	return label.equals(v.label());
    }

    /**
     * Return a hashcode associated with the vertex.
     *
     * @post returns hash code for vertex
     * 
     * @return An integer for use in hashing values into tables.
     */
    public int hashCode()
    {
	return label.hashCode();
    }

    /**
     * Construct a string representing vertex.
     *
     * @post returns string representation of vertex
     * 
     * @return A string representing vertex.
     */
    public String toString()
    {
	return "<Vertex: "+label+">";
    }
}