package structure;import java.util.Map;/** * An implementation of the the java.util.Map.Entry interface, Entry * is a simple key value pair, from which both the key and the value * can be accessed. {@link structure.Association} and related classes * also implement the Map interface and have expanded functionality.  * <P> * Typical Usage: * <P> * <pre> * ... *     Entry e = new {@link #Entry(Object,Object) Entry(aKey, aValue)}; *     Object key = e.{@link #getKey()}; *     Object value = e.{@link #getValue()}; *     e.{@link #setValue(Object) setValue(newValue)}; * ... * </pre> *  */public class Entry implements java.util.Map.Entry{    protected Object theKey; // the key of the key-value pair    /**     * The mutable value.  An arbitrary object.     */    protected Object theValue; // the value of the key-value pair    /**     * Constructs a pair from a key and value.     *     * @pre Key is non-null     * @post Constructs a key-value pair     *      * @param key A non-null object.     * @param value A (possibly null) object.     */    public Entry(Object key, Object value)    {	Assert.pre(key != null, "Key must not be null.");	theKey = key;	theValue = value;    }    /**     * Constructs a pair from a key; value is null.     *     * @pre Key is non-null     * @post Constructs a key-value pair     *      * @param key A non-null key value.     */    public Entry(Object key)    {	this(key,null);    }    /**     * Standard comparison function.  Comparison based on keys only.     *     * @pre Other is non-null Entry     * @post Returns true iff the keys are equal     *      * @param other Another association.     * @return True iff the keys are equal.     */    public boolean equals(Object other)    {	Map.Entry otherEntry = (Map.Entry)other;	return getKey().equals(otherEntry.getKey());    }        /**     * Standard hashcode function.     *     * @post Return hash code association with this association     *      * @return A hash code for association.     * @see Hashtable     */    public int hashCode()    {	return getKey().hashCode();    }        /**     * Fetch value from association.  May return null.     *     * @post Returns value from association     *      * @return The value field of the association.     */    public Object getValue()    {	return theValue;    }    /**     * Fetch key from association.  Should not return null.     *     * @post Returns key from association     *      * @return Key of the key-value pair.     */    public Object getKey()    {	return theKey;    }    /**     * Sets the value of the key-value pair.     *     * @post Sets association's value to value     *      * @param value The new value.     */    public Object setValue(Object value)    {	Object result = theValue;	theValue = value;	return result;    }    /**     * Standard string representation of an association.     *     * @post Returns string representation     *      * @return String representing key-value pair.     */    public String toString()    {	StringBuffer s = new StringBuffer();	s.append("<Entry: "+getKey()+"="+getValue()+">");	return s.toString();    }}