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With inspiration from other libraries

There is a newer version: 2.12.15

/**
 * Logback: the reliable, generic, fast and flexible logging framework.
 * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
 *
 * This program and the accompanying materials are dual-licensed under
 * either the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation
 *
 *   or (per the licensee's choosing)
 *
 * under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation.
 */
package ch.qos.logback.core.pattern;

/**
 * A minimal converter which sets up the general interface for derived classes. 
 * It also implements the functionality to chain converters in a linked list.
 * 
 * @author ceki
 */
abstract public class Converter {

    Converter next;

    /**
     * The convert method is responsible for extracting data from the event and
     * storing it for later use by the write method.
     * 
     * @param event
     */
    public abstract String convert(E event);

    /**
     * In its simplest incarnation, a convert simply appends the data extracted from
     * the event to the buffer passed as parameter.
     * 
     * @param buf The input buffer where data is appended
     * @param event The event from where data is extracted
     */
    public void write(StringBuilder buf, E event) {
        buf.append(convert(event));
    }

    public final void setNext(Converter next) {
        if (this.next != null) {
            throw new IllegalStateException("Next converter has been already set");
        }
        this.next = next;
    }

    public final Converter getNext() {
        return next;
    }
}