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A Java library for reading and writing iCalendar (*.ics) files
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/**
 * Copyright (c) 2012, Ben Fortuna
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *  o Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *
 *  o Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 *
 *  o Neither the name of Ben Fortuna nor the names of any other contributors
 * may be used to endorse or promote products derived from this software
 * without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package net.fortuna.ical4j.util;

import java.util.TimeZone;

/**
 * $Id$ [5/07/2004]
 *
 * Utility methods relevant to Java timezones.
 *
 * @author Ben Fortuna
 */
public final class TimeZones {
    
    /**
     * The timezone identifier for UTC time.
     */
    public static final String UTC_ID = "Etc/UTC";
    
    /**
     * The timezone identifier for UTC time in the IBM JVM.
     */
    public static final String IBM_UTC_ID = "GMT";
    
    /**
     * The timezone identifier for GMT time.
     */
    public static final String GMT_ID = "Etc/GMT";

    private static final TimeZone UTC_TIMEZONE;
    static {
        UTC_TIMEZONE = TimeZone.getTimeZone(UTC_ID);
    }

    /**
     * Constructor made private to enforce static nature.
     */
    private TimeZones() {
    }
    
    /**
     * Indicates whether the specified timezone is equivalent to
     * UTC time.
     * @param timezone a timezone instance
     * @return true if the timezone is UTC time, otherwise false
     */
    public static boolean isUtc(final TimeZone timezone) {
//        return timezone.hasSameRules(TimeZone.getTimeZone(UTC_ID));
//        return timezone.getRawOffset() == 0;
        return UTC_ID.equals(timezone.getID())
            || IBM_UTC_ID.equals(timezone.getID());
    }

    public static TimeZone getDefault() {
        if ("true".equals(Configurator.getProperty("net.fortuna.ical4j.timezone.default.utc").orElse("false"))) {
            return getUtcTimeZone();
        }
        return TimeZone.getDefault();
    }

    /**
     * Although timezones are not really applicable to DATE instances in iCalendar, the implementation
     * in iCal4j requires the use of a timezone. Dates in iCal4j may be either "floating", in that they
     * use the default Java timezone, or alternatively will use UTC (this is the default).
     * 
     * The use of floating dates may be configured by specifying the following as a system property or in
     * a file called "ical4j.properties" in the classpath:
     * 
     * net.fortuna.ical4j.timezone.date.floating=true
     * 
     * @return the timezone used for date instances
     */
    public static TimeZone getDateTimeZone() {
    	if ("true".equals(Configurator.getProperty("net.fortuna.ical4j.timezone.date.floating").orElse("false"))) {
    		return TimeZone.getDefault();
    	}
    	return getUtcTimeZone();
    }

    /**
     * Get the UTC Timezone.
     */
    public static TimeZone getUtcTimeZone() {
        return UTC_TIMEZONE;
    }
}