element.java

发布/订阅系统路由重配算法,可应用于ad hoc环境

/*
 * project: RebecaSim
 * package: graph
 * file:    Element.java
 * version: 0.1
 * date:    01.04.2005
 * 
 * This software is part of the diploma thesis "Ein adaptives Brokernetz 
 * für Publish/Subscribe Systeme".
 */
package graph;

import util.*;

/**
 * The <code>Element</code> class is the superclass of many 
 * classes in the <code>graph</code> package, since nearly everything
 * is an element.
 * The implementation ensures that each element has an unique ID and
 * provides the functionality to easily attach arbitary properties mapped
 * to belonging values. 
 * 
 * @version 0.1 01.04.2005
 * @author  Helge Parzyjegla
 */
public class Element implements Comparable{

	/** The unique ID of this element. */
	public final ID id;
	
	/** A hashtable to store property to value mappings. */
	protected SimpleHashtable properties;
	
	/** 
	 * Constructs a new element with an unique ID and without any
	 * attached properties.
	 */ 
	public Element() {
		this.id = new ID();
		this.properties = new SimpleHashtable();
	}
	
	/**
	 * Compares this element to the specified object.
	 * The result is <code>true</code> if and only if the object 
	 * <code>o<code> is not <code>null</code> and is also an element
	 * with the same id.
	 * 
	 * @param  o the object to compare to.
	 * @return <code>true</code> if the specified object is not 
	 *         <code>null</code> and is also an element with the same id.
	 * @throws NullPointerException if <code>o</code> is <code>null</code>.
	 */
	public boolean equals(Object o) {
		if (o instanceof Element) {
			return this.id.equals(((Element)o).id); 
		}
		return false;
	}
	
	/**
	 * Returns a hash code for this <code>element</code> based on its id.
	 * 
	 * @return a hash code value for this <code>element</code>, equal to
	 *         its <code>ID</code>s hash code.
	 */
	public int hashCode() {
		return id.hashCode();
	}
	
	/**
	 * Attaches a <code>property</code> to <code>value</code> mapping 
	 * to this element.
	 * The property can not be <code>null<code>. <p>
	 *  
	 * Use the <code>getProperty</code> method with an equal key 
	 * to retrieve the mapped value again.
	 * 
	 * @param  property the property. 
	 * @param  value    the belonging value.
	 * @return the previous value of the specified property or 
	 *         <code>null</code> if the property was not attached before.
	 * @throws NullPointerException if property is <code>null<code>.
	 * @see    #getProperty(Property)
	 */
	public synchronized Object setProperty(Property property, Object value) {
		if (property == null) {
			throw new NullPointerException("Null is not a valid property.");
		}
		property.addElement(this);
		return properties.put(property.getKey(), value);
	}

	/**
	 * Retrives the value the attached property is mapped to.
	 * 
	 * @param  property an attached property.
	 * @return the value to which the property is mapped or <code>null</code>
	 *         if the property is not attached to this element.
	 * @throws NullPointerException if property is <code>null<code>.
	 * @see    #setProperty(Property)
	 */
	public synchronized Object getProperty(Property property){
		if (property == null) {
			throw new NullPointerException("Null is not a valid property.");
		}
		return properties.get(property.getKey());
	}

	/**
	 * Tests whether the element has a value mapped by that property.
	 * @param property the property.
	 * @return true if a value is attached for this property.
	 */
	public synchronized boolean hasProperty(Property property){
		if (property == null) {
			throw new NullPointerException("Null is not a valid property.");
		}
		return properties.containsKey(property.getKey());
	}
	
	/**
	 * Removes the property (and its corresponding value) from this element.
	 * This method does nothing if the property is not attached to this
	 * element.
	 * 
	 * @param  property an attached property.
	 * @return the value to which the property was mapped or <code>null</code>
	 *         if the property is not attached to this element.
	 * @throws NullPointerException if property is <code>null<code>.
	 */
	public synchronized Object removeProperty(Property property){
		if (property == null) {
			throw new NullPointerException("Null is not a valid property.");
		}
		property.removeElement(this);
		return properties.remove(property.getKey());
	}

	/**
	 * Compares this <code>Element</code> object to another object.
	 * The elements ordering is inherited from its ids.
	 * 
	 * @param  o the object to compare to.
	 * @return <code>0</code> if both elements are equal,
	 *         a value less than <code>0</code> if this element is less 
	 *         than the argument and 
	 *         a value greater than <code>0</code> if this element is greater 
	 *         than the argument.
	 * @throws NullPointerException if the argument is <code>null</code>.
	 * @throws ClassCastException if the argument is not an 
	 *         <code>Element</code>.   
	 */
	public int compareTo(Object o){
		return id.compareTo(((Element)o).id);
	}
}