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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 2000-2020, 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.]
 *
 * --------------------
 * HexNumberFormat.java
 * --------------------
 * (C) Copyright 2007-2020, by Richard West and Contributors.
 *
 * Original Author:  Richard West, Advanced Micro Devices, Inc.;
 * Contributor(s):   David Gilbert (for Object Refinery Limited);
 *
 */

package org.jfree.chart.util;

import java.text.FieldPosition;
import java.text.NumberFormat;
import java.text.ParsePosition;

/**
 * A custom number formatter that formats numbers as hexadecimal strings.
 * There are some limitations, so be careful using this class.
 *
 * @since 1.0.6
 */
public class HexNumberFormat extends NumberFormat {

    /** Number of hexadecimal digits for a byte. */
    public static final int BYTE = 2;

    /** Number of hexadecimal digits for a word. */
    public static final int WORD = 4;

    /** Number of hexadecimal digits for a double word. */
    public static final int DWORD = 8;

    /** Number of hexadecimal digits for a quad word. */
    public static final int QWORD = 16;

    /** The number of digits (shorter strings will be left padded). */
    private int m_numDigits = DWORD;

    /**
     * Creates a new instance with 8 digits.
     */
    public HexNumberFormat() {
        this(DWORD);
    }

    /**
     * Creates a new instance with the specified number of digits.

     * @param digits  the digits.
     */
    public HexNumberFormat(int digits) {
        super();
        this.m_numDigits = digits;
    }

    /**
     * Returns the number of digits.
     *
     * @return The number of digits.
     */
    public final int getNumberOfDigits() {
        return this.m_numDigits;
    }

    /**
     * Sets the number of digits.
     *
     * @param digits  the number of digits.
     */
    public void setNumberOfDigits(int digits) {
        this.m_numDigits = digits;
    }

    /**
     * Formats the specified number as a hexadecimal string.  The decimal
     * fraction is ignored.
     *
     * @param number  the number to format.
     * @param toAppendTo  the buffer to append to (ignored here).
     * @param pos  the field position (ignored here).
     *
     * @return The string buffer.
     */
    @Override
    public StringBuffer format(double number, StringBuffer toAppendTo,
            FieldPosition pos) {
        return format((long) number, toAppendTo, pos);
    }

    /**
     * Formats the specified number as a hexadecimal string.  The decimal
     * fraction is ignored.
     *
     * @param number  the number to format.
     * @param toAppendTo  the buffer to append to (ignored here).
     * @param pos  the field position (ignored here).
     *
     * @return The string buffer.
     */
    @Override
    public StringBuffer format(long number, StringBuffer toAppendTo,
            FieldPosition pos) {
        String l_hex = Long.toHexString(number).toUpperCase();

        int l_pad = this.m_numDigits - l_hex.length();
        l_pad = (0 < l_pad) ? l_pad : 0;

        StringBuffer l_extended = new StringBuffer("0x");
        for (int i = 0; i < l_pad; i++) {
            l_extended.append(0);
        }
        l_extended.append(l_hex);

        return l_extended;
    }

    /**
     * Parsing is not implemented, so this method always returns
     * {@code null}.
     *
     * @param source  ignored.
     * @param parsePosition  ignored.
     *
     * @return Always {@code null}.
     */
    @Override
    public Number parse (String source, ParsePosition parsePosition) {
        return null; // don't bother with parsing
    }

}