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JFreeChart is a class library, written in Java, for generating charts. Utilising the Java2D APIs, it currently supports bar charts, pie charts, line charts, XY-plots and time series plots.
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/* ===========================================================
 * JFreeChart : a free chart library for the Java(tm) platform
 * ===========================================================
 *
 * (C) Copyright 2000-2013, by Object Refinery Limited and Contributors.
 *
 * Project Info:  http://www.jfree.org/jfreechart/index.html
 *
 * This library is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or
 * (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
 * License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
 * USA.
 *
 * [Oracle and Java are registered trademarks of Oracle and/or its affiliates. 
 * Other names may be trademarks of their respective owners.]
 *
 * -----------------------------
 * DefaultKeyedValueDataset.java
 * -----------------------------
 * (C) Copyright 2003-2008, by Object Refinery Limited.
 *
 * Original Author:  David Gilbert (for Object Refinery Limited);
 * Contributor(s):   -;
 *
 * Changes
 * -------
 * 27-Mar-2003 : Version 1 (DG);
 * 18-Aug-2003 : Implemented Cloneable (DG);
 *
 */

package org.jfree.data.general;

import java.io.Serializable;

import org.jfree.data.DefaultKeyedValue;
import org.jfree.data.KeyedValue;
import org.jfree.util.ObjectUtilities;

/**
 * A default implementation of the {@link KeyedValueDataset} interface.
 */
public class DefaultKeyedValueDataset extends AbstractDataset
        implements KeyedValueDataset, Serializable {

    /** For serialization. */
    private static final long serialVersionUID = -8149484339560406750L;

    /** Storage for the data. */
    private KeyedValue data;

    /**
     * Constructs a new dataset, initially empty.
     */
    public DefaultKeyedValueDataset() {
        this(null);
    }

    /**
     * Creates a new dataset with the specified initial value.
     *
     * @param key  the key.
     * @param value  the value (null permitted).
     */
    public DefaultKeyedValueDataset(Comparable key, Number value) {
        this(new DefaultKeyedValue(key, value));
    }

    /**
     * Creates a new dataset that uses the data from a {@link KeyedValue}
     * instance.
     *
     * @param data  the data (null permitted).
     */
    public DefaultKeyedValueDataset(KeyedValue data) {
        this.data = data;
    }

    /**
     * Returns the key associated with the value, or null if the
     * dataset has no data item.
     *
     * @return The key.
     */
    @Override
    public Comparable getKey() {
        Comparable result = null;
        if (this.data != null) {
            result = this.data.getKey();
        }
        return result;
    }

    /**
     * Returns the value.
     *
     * @return The value (possibly null).
     */
    @Override
    public Number getValue() {
        Number result = null;
        if (this.data != null) {
            result = this.data.getValue();
        }
        return result;
    }

    /**
     * Updates the value.
     *
     * @param value  the new value (null permitted).
     */
    public void updateValue(Number value) {
        if (this.data == null) {
            throw new RuntimeException("updateValue: can't update null.");
        }
        setValue(this.data.getKey(), value);
    }

    /**
     * Sets the value for the dataset and sends a {@link DatasetChangeEvent} to
     * all registered listeners.
     *
     * @param key  the key.
     * @param value  the value (null permitted).
     */
    public void setValue(Comparable key, Number value) {
        this.data = new DefaultKeyedValue(key, value);
        notifyListeners(new DatasetChangeEvent(this, this));
    }

    /**
     * Tests this dataset for equality with an arbitrary object.
     *
     * @param obj  the object (null permitted).
     *
     * @return A boolean.
     */
    @Override
    public boolean equals(Object obj) {
        if (obj == this) {
            return true;
        }
        if (!(obj instanceof KeyedValueDataset)) {
            return false;
        }
        KeyedValueDataset that = (KeyedValueDataset) obj;
        if (this.data == null) {
            if (that.getKey() != null || that.getValue() != null) {
                return false;
            }
            return true;
        }
        if (!ObjectUtilities.equal(this.data.getKey(), that.getKey())) {
            return false;
        }
        if (!ObjectUtilities.equal(this.data.getValue(), that.getValue())) {
            return false;
        }
        return true;
    }

    /**
     * Returns a hash code.
     *
     * @return A hash code.
     */
    @Override
    public int hashCode() {
        return (this.data != null ? this.data.hashCode() : 0);
    }

    /**
     * Creates a clone of the dataset.
     *
     * @return A clone.
     *
     * @throws CloneNotSupportedException This class will not throw this
     *         exception, but subclasses (if any) might.
     */
    @Override
    public Object clone() throws CloneNotSupportedException {
        DefaultKeyedValueDataset clone
                = (DefaultKeyedValueDataset) super.clone();
        return clone;
    }

}