src/main/java/org/apache/commons/configuration2/event/Event.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.configuration2.event;

import java.util.EventObject;

/**
 * <p>
 * The base class for all events generated by this library.
 * </p>
 * <p>
 * The events produced by objects in this library are arranged in an inheritance
 * hierarchy. This base class defines some basic properties common to all
 * configuration events. Especially, an event has an {@link EventType} which
 * describes its semantics. The event type can also be used for filtering for
 * events or for defining event listeners on a fine-grained basis.
 * </p>
 *
 * @version $Id$
 * @since 2.0
 */
public class Event extends EventObject
{
    /**
     * The root event type for all configuration-related events. All specific
     * event types have this type as super direct (directly or indirectly).
     */
    public static final EventType<Event> ANY =
            new EventType<Event>(null, "ANY");

    /**
     * Constant for the format used in toString() for a property representation.
     */
    private static final String FMT_PROPERTY = " %s=%s";

    /**
     * Constant for the initial buffer size for the generation of the string
     * representation.
     */
    private static final int BUF_SIZE = 256;

    /** The type of this event. */
    private final EventType<? extends Event> eventType;

    /**
     * Creates a new instance of {@code Event} and sets basic properties.
     *
     * @param source the object on which the Event initially occurred (must not
     *        be <b>null</b>)
     * @param evType the type of this event (must not be <b>null</b>)
     * @throws IllegalArgumentException if a required parameter is null
     */
    public Event(Object source, EventType<? extends Event> evType)
    {
        super(source);
        if (evType == null)
        {
            throw new IllegalArgumentException("Event type must not be null!");
        }
        eventType = evType;
    }

    /**
     * Returns the type of this event.
     *
     * @return the event type
     */
    public EventType<? extends Event> getEventType()
    {
        return eventType;
    }

    /**
     * Returns a string representation for this object. This string contains the
     * event class and a list of all properties.
     *
     * @return a string for this object
     */
    @Override
    public String toString()
    {
        StringBuilder buf = new StringBuilder(BUF_SIZE);
        buf.append(getClass().getSimpleName());
        buf.append(" [");
        appendPropertyRepresentation(buf, "source", getSource());
        appendPropertyRepresentation(buf, "eventType", getEventType());
        buf.append(" ]");
        return buf.toString();
    }

    /**
     * Helper method for appending a representation for a property to the
     * overall string representation for this object. This method is called by
     * {@code toString()} for generating string fragments for the properties of
     * this class. It can also be used by derived classes which extend the
     * string representation of this base class.
     *
     * @param buf the target buffer
     * @param property the name of the property
     * @param value the property value
     */
    protected void appendPropertyRepresentation(StringBuilder buf,
            String property, Object value)
    {
        buf.append(String.format(FMT_PROPERTY, property, String.valueOf(value)));
    }
}