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JCommon is a free general purpose Java class library that is used in several projects at www.jfree.org, including JFreeChart and JFreeReport.
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/* ========================================================================
 * JCommon : a free general purpose class library for the Java(tm) platform
 * ========================================================================
 *
 * (C) Copyright 2000-2005, by Object Refinery Limited and Contributors.
 * 
 * Project Info:  http://www.jfree.org/jcommon/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.]
 * 
 * ---------------------------
 * LengthLimitingDocument.java
 * ---------------------------
 * (C)opyright 2003, 2004, by Thomas Morgner and Contributors.
 *
 * Original Author:  Thomas Morgner;
 * Contributor(s):   David Gilbert (for Object Refinery Limited);
 *
 * $Id: LengthLimitingDocument.java,v 1.3 2005/10/18 13:18:34 mungady Exp $
 *
 * Changes
 * -------
 * 22-Jan-2003 : Initial version
 * 05-Feb-2003 : Documentation
 * 
 */

package org.jfree.ui;

import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.PlainDocument;

/**
 * This Document restricts the size of the contained plain text to the given number of
 * characters.
 *
 * @author Thomas Morgner
 */
public class LengthLimitingDocument extends PlainDocument {

    /** The maximum length. */
    private int maxlen;

    /**
     * Creates a new LengthLimitingDocument, with no limitation.
     */
    public LengthLimitingDocument() {
        this(-1);
    }

    /**
     * Creates a new LengthLimitingDocument with the given limitation. No more than
     * maxlen characters can be added to the document. If maxlen is negative, then
     * no length check is performed.
     *
     * @param maxlen the maximum number of elements in this document
     */
    public LengthLimitingDocument(final int maxlen) {
        super();
        this.maxlen = maxlen;
    }

    /**
     * Sets the maximum number of characters for this document. Existing characters
     * are not removed.
     *
     * @param maxlen the maximum number of characters in this document.
     */
    public void setMaxLength(final int maxlen) {
        this.maxlen = maxlen;
    }

    /**
     * Returns the defined maximum number characters for this document.
     * @return the maximum number of characters
     */
    public int getMaxLength() {
        return this.maxlen;
    }

    /**
     * Inserts the string into the document. If the length of the document would
     * violate the maximum characters restriction, then the string is cut down so
     * that
     * @param offs the offset, where the string should be inserted into the document
     * @param str the string that should be inserted
     * @param a the attribute set assigned for the document
     * @throws javax.swing.text.BadLocationException if the offset is not correct
     */
    public void insertString(final int offs, final String str, final AttributeSet a)
        throws BadLocationException {
        if (str == null) {
            return;
        }

        if (this.maxlen < 0) {
            super.insertString(offs, str, a);
        }

        final char[] numeric = str.toCharArray();
        final StringBuffer b = new StringBuffer();
        b.append(numeric, 0, Math.min(this.maxlen, numeric.length));
        super.insertString(offs, b.toString(), a);
    }
    
}