EventType.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.io.Serializable;
import java.util.HashSet;
import java.util.Set;

/**
 * <p>
 * A class representing an event type.
 * </p>
 * <p>
 * The events produced by <em>Commons Configuration</em> all have a specific type. The event type can be used to
 * determine the meaning of a specific event. It also acts as filter criterion when event listeners are registered. The
 * listener is then called only for events of this type or derived types. The events in this library form a natural
 * hierarchy with base types and more specialized types. By specifying an appropriate event type at listener
 * registration time, it can be determined on a fine-granular basis which events are propagated to the listener.
 * </p>
 * <p>
 * Note: Users familiar with JavaFX probably recognize this approach to event handling. It allows for generic event
 * listener interfaces and a natural selection of events to be processed.
 * </p>
 *
 * @param <T> the event associated with this type
 * @since 2.0
 */
public class EventType<T extends Event> implements Serializable {
    /** Serial version UID. */
    private static final long serialVersionUID = 20150416L;

    /** Constant for the format used by toString(). */
    private static final String FMT_TO_STRING = "%s [ %s ]";

    /**
     * Returns a set with all event types that are super types of the specified type. This set contains the direct and
     * indirect super types and also includes the given type itself. The passed in type may be <strong>null</strong>, then an empty
     * set is returned.
     *
     * @param eventType the event type in question
     * @return a set with all super event types
     */
    public static Set<EventType<?>> fetchSuperEventTypes(final EventType<?> eventType) {
        final Set<EventType<?>> types = new HashSet<>();
        EventType<?> currentType = eventType;
        while (currentType != null) {
            types.add(currentType);
            currentType = currentType.getSuperType();
        }
        return types;
    }

    /**
     * Checks whether an event type is derived from another type. This implementation tests whether {@code baseType} is a
     * direct or indirect super type of {@code derivedType}. If one of the types is <strong>null</strong>, result is <strong>false</strong>.
     *
     * @param derivedType the derived event type
     * @param baseType the base event type
     * @return <strong>true</strong> if the derived type is an instance of the base type, <strong>false</strong> otherwise
     */
    public static boolean isInstanceOf(final EventType<?> derivedType, final EventType<?> baseType) {
        EventType<?> currentType = derivedType;
        while (currentType != null) {
            if (currentType == baseType) {
                return true;
            }
            currentType = currentType.getSuperType();
        }
        return false;
    }

    /** Stores the super type of this type. */
    private final EventType<? super T> superType;

    /** A name for this event type. */
    private final String name;

    /**
     * Creates a new instance of {@code EventType} and initializes it with the super type and a type name. If no super type
     * is specified, this is the root event type.
     *
     * @param superEventType the super event type
     * @param typeName the name of this event type
     */
    public EventType(final EventType<? super T> superEventType, final String typeName) {
        superType = superEventType;
        name = typeName;
    }

    /**
     * Gets the name of this event type. The name has no specific semantic meaning. It is just used for debugging
     * purposes and also part of the string representation of this event type.
     *
     * @return the event type name
     */
    public String getName() {
        return name;
    }

    /**
     * Gets the super event type. Result is <strong>null</strong> for the root event type.
     *
     * @return the super event type
     */
    public EventType<? super T> getSuperType() {
        return superType;
    }

    /**
     * Returns a string representation for this object. This method is mainly overridden for debugging purposes. The
     * returned string contains the name of this event type.
     *
     * @return a string for this object
     */
    @Override
    public String toString() {
        return String.format(FMT_TO_STRING, getClass().getSimpleName(), getName());
    }
}