public class PaintMap implements Cloneable, Serializable {

    /** For serialization. */
    static final long serialVersionUID = -4639833772123069274L;

    /** Storage for the keys and values. */
    private transient Map store;
    
    /**
     * Creates a new (empty) map.
     */
    public PaintMap() {
        this.store = new HashMap();    
    }
    
    /**
     * Returns the paint associated with the specified key, or 
     * <code>null</code>.
     * 
     * @param key  the key (<code>null</code> not permitted).
     * 
     * @return The paint, or <code>null</code>.
     * 
     * @throws IllegalArgumentException if <code>key</code> is 
     *     <code>null</code>.
     */
    public Paint getPaint(Comparable key) {
        if (key == null) {
            throw new IllegalArgumentException("Null 'key' argument.");
        }
        return (Paint) this.store.get(key);
    }
    
    /**
     * Returns <code>true</code> if the map contains the specified key, and
     * <code>false</code> otherwise.
     * 
     * @param key  the key.
     * 
     * @return <code>true</code> if the map contains the specified key, and
     * <code>false</code> otherwise.
     */
    public boolean containsKey(Comparable key) {
        return this.store.containsKey(key);
    }
    
    /**
     * Adds a mapping between the specified <code>key</code> and 
     * <code>paint</code> values.
     * 
     * @param key  the key (<code>null</code> not permitted).
     * @param paint  the paint.
     * 
     * @throws IllegalArgumentException if <code>key</code> is 
     *     <code>null</code>.
     */
    public void put(Comparable key, Paint paint) {
        if (key == null) {
            throw new IllegalArgumentException("Null 'key' argument.");
        }
        this.store.put(key, paint);
    }
    
    /**
     * Resets the map to empty.
     */
    public void clear() {
        this.store.clear();
    }
    
    /**
     * Tests this map for equality with an arbitrary object.
     * 
     * @param obj  the object (<code>null</code> permitted).
     * 
     * @return A boolean.
     */
    public boolean equals(Object obj) {
        if (obj == this) {
            return true;
        }
        if (!(obj instanceof PaintMap)) {
            return false;
        }
        PaintMap that = (PaintMap) obj;
        if (this.store.size() != that.store.size()) {
            return false;
        }
        Set keys = this.store.keySet();
        Iterator iterator = keys.iterator();
        while (iterator.hasNext()) {
            Comparable key = (Comparable) iterator.next();
            Paint p1 = getPaint(key);
            Paint p2 = that.getPaint(key);
            if (!PaintUtilities.equal(p1, p2)) {
                return false;
            }
        }
        return true;
    }
    
    /**
     * Returns a clone of this <code>PaintMap</code>.
     * 
     * @return A clone of this instance.
     * 
     * @throws CloneNotSupportedException if any key is not cloneable.
     */
    public Object clone() throws CloneNotSupportedException {
        // TODO: I think we need to make sure the keys are actually cloned,
        // whereas the paint instances are always immutable so they're OK
        return super.clone();
    }
    
    /**
     * Provides serialization support.
     *
     * @param stream  the output stream.
     *
     * @throws IOException  if there is an I/O error.
     */
    private void writeObject(ObjectOutputStream stream) throws IOException {
        stream.defaultWriteObject();
        stream.writeInt(this.store.size());
        Set keys = this.store.keySet();
        Iterator iterator = keys.iterator();
        while (iterator.hasNext()) {
            Comparable key = (Comparable) iterator.next();
            stream.writeObject(key);
            Paint paint = getPaint(key);
            SerialUtilities.writePaint(paint, stream);
        }
    }

    /**
     * Provides serialization support.
     *
     * @param stream  the input stream.
     *
     * @throws IOException  if there is an I/O error.
     * @throws ClassNotFoundException  if there is a classpath problem.
     */
    private void readObject(ObjectInputStream stream) 
            throws IOException, ClassNotFoundException {
        stream.defaultReadObject();
        this.store = new HashMap();
        int keyCount = stream.readInt();
        for (int i = 0; i < keyCount; i++) {
            Comparable key = (Comparable) stream.readObject();
            Paint paint = SerialUtilities.readPaint(stream);
            this.store.put(key, paint);