001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *     http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.configuration2.event;
018
019import java.io.Serializable;
020import java.util.HashSet;
021import java.util.Set;
022
023/**
024 * <p>
025 * A class representing an event type.
026 * </p>
027 * <p>
028 * The events produced by <em>Commons Configuration</em> all have a specific type. The event type can be used to
029 * determine the meaning of a specific event. It also acts as filter criterion when event listeners are registered. The
030 * listener is then called only for events of this type or derived types. The events in this library form a natural
031 * hierarchy with base types and more specialized types. By specifying an appropriate event type at listener
032 * registration time, it can be determined on a fine-granular basis which events are propagated to the listener.
033 * </p>
034 * <p>
035 * Note: Users familiar with JavaFX probably recognize this approach to event handling. It allows for generic event
036 * listener interfaces and a natural selection of events to be processed.
037 * </p>
038 *
039 * @since 2.0
040 * @param <T> the event associated with this type
041 */
042public class EventType<T extends Event> implements Serializable {
043    /** Serial version UID. */
044    private static final long serialVersionUID = 20150416L;
045
046    /** Constant for the format used by toString(). */
047    private static final String FMT_TO_STRING = "%s [ %s ]";
048
049    /** Stores the super type of this type. */
050    private final EventType<? super T> superType;
051
052    /** A name for this event type. */
053    private final String name;
054
055    /**
056     * Creates a new instance of {@code EventType} and initializes it with the super type and a type name. If no super type
057     * is specified, this is the root event type.
058     *
059     * @param superEventType the super event type
060     * @param typeName the name of this event type
061     */
062    public EventType(final EventType<? super T> superEventType, final String typeName) {
063        superType = superEventType;
064        name = typeName;
065    }
066
067    /**
068     * Gets the super event type. Result is <b>null</b> for the root event type.
069     *
070     * @return the super event type
071     */
072    public EventType<? super T> getSuperType() {
073        return superType;
074    }
075
076    /**
077     * Gets the name of this event type. The name has no specific semantic meaning. It is just used for debugging
078     * purposes and also part of the string representation of this event type.
079     *
080     * @return the event type name
081     */
082    public String getName() {
083        return name;
084    }
085
086    /**
087     * Returns a string representation for this object. This method is mainly overridden for debugging purposes. The
088     * returned string contains the name of this event type.
089     *
090     * @return a string for this object
091     */
092    @Override
093    public String toString() {
094        return String.format(FMT_TO_STRING, getClass().getSimpleName(), getName());
095    }
096
097    /**
098     * Returns a set with all event types that are super types of the specified type. This set contains the direct and
099     * indirect super types and also includes the given type itself. The passed in type may be <b>null</b>, then an empty
100     * set is returned.
101     *
102     * @param eventType the event type in question
103     * @return a set with all super event types
104     */
105    public static Set<EventType<?>> fetchSuperEventTypes(final EventType<?> eventType) {
106        final Set<EventType<?>> types = new HashSet<>();
107        EventType<?> currentType = eventType;
108        while (currentType != null) {
109            types.add(currentType);
110            currentType = currentType.getSuperType();
111        }
112        return types;
113    }
114
115    /**
116     * Checks whether an event type is derived from another type. This implementation tests whether {@code baseType} is a
117     * direct or indirect super type of {@code derivedType}. If one of the types is <b>null</b>, result is <b>false</b>.
118     *
119     * @param derivedType the derived event type
120     * @param baseType the base event type
121     * @return <b>true</b> if the derived type is an instance of the base type, <b>false</b> otherwise
122     */
123    public static boolean isInstanceOf(final EventType<?> derivedType, final EventType<?> baseType) {
124        EventType<?> currentType = derivedType;
125        while (currentType != null) {
126            if (currentType == baseType) {
127                return true;
128            }
129            currentType = currentType.getSuperType();
130        }
131        return false;
132    }
133}