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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 2000-2008, 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.
 *
 * [Java is a trademark or registered trademark of Sun Microsystems, Inc.
 * in the United States and other countries.]
 *
 * -------------------
 * GrayPaintScale.java
 * -------------------
 * (C) Copyright 2006-2008, by Object Refinery Limited.
 *
 * Original Author:  David Gilbert (for Object Refinery Limited);
 * Contributor(s):   -;
 *
 * Changes
 * -------
 * 05-Jul-2006 : Version 1 (DG);
 * 31-Jan-2007 : Renamed min and max to lowerBound and upperBound (DG);
 * 26-Sep-2007 : Fixed bug 1767315, problem in getPaint() method (DG);
 *
 */

package org.jfree.chart.renderer;

import java.awt.Color;
import java.awt.Paint;
import java.io.Serializable;

import org.jfree.util.PublicCloneable;

/**
 * A paint scale that returns shades of gray.
 *
 * @since 1.0.4
 */
public class GrayPaintScale
        implements PaintScale, PublicCloneable, Serializable {

    /** The lower bound. */
    private double lowerBound;

    /** The upper bound. */
    private double upperBound;

    /**
     * Creates a new GrayPaintScale instance with default values.
     */
    public GrayPaintScale() {
        this(0.0, 1.0);
    }

    /**
     * Creates a new paint scale for values in the specified range.
     *
     * @param lowerBound  the lower bound.
     * @param upperBound  the upper bound.
     *
     * @throws IllegalArgumentException if lowerBound is not
     *       less than upperBound.
     */
    public GrayPaintScale(double lowerBound, double upperBound) {
        if (lowerBound >= upperBound) {
            throw new IllegalArgumentException(
                    "Requires lowerBound < upperBound.");
        }
        this.lowerBound = lowerBound;
        this.upperBound = upperBound;
    }

    /**
     * Returns the lower bound.
     *
     * @return The lower bound.
     *
     * @see #getUpperBound()
     */
    public double getLowerBound() {
        return this.lowerBound;
    }

    /**
     * Returns the upper bound.
     *
     * @return The upper bound.
     *
     * @see #getLowerBound()
     */
    public double getUpperBound() {
        return this.upperBound;
    }

    /**
     * Returns a paint for the specified value.
     *
     * @param value  the value (must be within the range specified by the
     *         lower and upper bounds for the scale).
     *
     * @return A paint for the specified value.
     */
    public Paint getPaint(double value) {
        double v = Math.max(value, this.lowerBound);
        v = Math.min(v, this.upperBound);
        int g = (int) ((v - this.lowerBound) / (this.upperBound
                - this.lowerBound) * 255.0);
        return new Color(g, g, g);
    }

    /**
     * Tests this GrayPaintScale instance for equality with an
     * arbitrary object.  This method returns true if and only
     * if:
     * 
     * obj is not null;
     * obj is an instance of GrayPaintScale;
     * 
     *
     * @param obj  the object (null permitted).
     *
     * @return A boolean.
     */
    public boolean equals(Object obj) {
        if (obj == this) {
            return true;
        }
        if (!(obj instanceof GrayPaintScale)) {
            return false;
        }
        GrayPaintScale that = (GrayPaintScale) obj;
        if (this.lowerBound != that.lowerBound) {
            return false;
        }
        if (this.upperBound != that.upperBound) {
            return false;
        }
        return true;
    }

    /**
     * Returns a clone of this GrayPaintScale instance.
     *
     * @return A clone.
     *
     * @throws CloneNotSupportedException if there is a problem cloning this
     *     instance.
     */
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

}