http://www.joda.org/joda-time/: Date and time library to replace JDK date handling (Joda.org) 
/*
 *  Copyright 2001-2005 Stephen Colebourne
 *
 *  Licensed 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.joda.time;


Defines a time period specified in terms of individual duration fields
such as years and days.


The implementation of this interface may be mutable or immutable. This
interface only gives access to retrieve data, never to change it.


Periods are split up into multiple fields, for example days and seconds.
Implementations are not required to evenly distribute the values across the fields.
The value for each field may be positive or negative.


When a time period is added to an instant, the effect is to add each field in turn.
For example, a time period could be defined as 3 months, 2 days and -1 hours.
In most circumstances this would be the same as 3 months, 1 day, and 23 hours.
However, when adding across a daylight savings boundary, a day may be 23 or 25 hours long.
Thus, the time period is always added field by field to the datetime.


Periods are independent of chronology, and can only be treated as durations
when paired with a time via an interval.

Author:Brian S O'Neill, Stephen Colebourne
See Also:
Since:1.0
/**
 * Defines a time period specified in terms of individual duration fields
 * such as years and days.
 * <p>
 * The implementation of this interface may be mutable or immutable. This
 * interface only gives access to retrieve data, never to change it.
 * <p>
 * Periods are split up into multiple fields, for example days and seconds.
 * Implementations are not required to evenly distribute the values across the fields.
 * The value for each field may be positive or negative.
 * <p>
 * When a time period is added to an instant, the effect is to add each field in turn.
 * For example, a time period could be defined as 3 months, 2 days and -1 hours.
 * In most circumstances this would be the same as 3 months, 1 day, and 23 hours.
 * However, when adding across a daylight savings boundary, a day may be 23 or 25 hours long.
 * Thus, the time period is always added field by field to the datetime.
 * <p>
 * Periods are independent of chronology, and can only be treated as durations
 * when paired with a time via an interval.
 *
 * @see ReadableDuration
 * @see ReadableInterval
 * @author Brian S O'Neill
 * @author Stephen Colebourne
 * @since 1.0
 */
public interface ReadablePeriod {

    
Gets the period type that defines which fields are included in the period.

Returns:the period type
/**
     * Gets the period type that defines which fields are included in the period.
     *
     * @return the period type
     */
    PeriodType getPeriodType();

    
Gets the number of fields that this period supports.

Returns:the number of fields supported
/**
     * Gets the number of fields that this period supports.
     *
     * @return the number of fields supported
     */
    int size();

    
Gets the field type at the specified index.

Params:
  • index –  the index to retrieve
Throws:
Returns:the field at the specified index
/**
     * Gets the field type at the specified index.
     *
     * @param index  the index to retrieve
     * @return the field at the specified index
     * @throws IndexOutOfBoundsException if the index is invalid
     */
    DurationFieldType getFieldType(int index);

    
Gets the value at the specified index.

Params:
  • index –  the index to retrieve
Throws:
Returns:the value of the field at the specified index
/**
     * Gets the value at the specified index.
     *
     * @param index  the index to retrieve
     * @return the value of the field at the specified index
     * @throws IndexOutOfBoundsException if the index is invalid
     */
    int getValue(int index);

    
Gets the value of one of the fields.


If the field type specified is not supported by the period then zero
is returned.

Params:
  • field –  the field type to query, null returns zero
Returns:the value of that field, zero if field not supported
/**
     * Gets the value of one of the fields.
     * <p>
     * If the field type specified is not supported by the period then zero
     * is returned.
     *
     * @param field  the field type to query, null returns zero
     * @return the value of that field, zero if field not supported
     */
    int get(DurationFieldType field);

    
Checks whether the field type specified is supported by this period.

Params:
  • field –  the field to check, may be null which returns false
Returns:true if the field is supported
/**
     * Checks whether the field type specified is supported by this period.
     *
     * @param field  the field to check, may be null which returns false
     * @return true if the field is supported
     */
    boolean isSupported(DurationFieldType field);

    //-----------------------------------------------------------------------
    
Get this period as an immutable Period object.


This will either typecast this instance, or create a new Period.

Returns:a Duration using the same field set and values
/**
     * Get this period as an immutable <code>Period</code> object.
     * <p>
     * This will either typecast this instance, or create a new <code>Period</code>.
     * 
     * @return a Duration using the same field set and values
     */
    Period toPeriod();

    
Get this object as a MutablePeriod.


This will always return a new MutablePeriod with the same fields.

Returns:a MutablePeriod using the same field set and values
/**
     * Get this object as a <code>MutablePeriod</code>.
     * <p>
     * This will always return a new <code>MutablePeriod</code> with the same fields.
     * 
     * @return a MutablePeriod using the same field set and values
     */
    MutablePeriod toMutablePeriod();

    //-----------------------------------------------------------------------
    
Compares this object with the specified object for equality based
on the value and type of each supported field.
All ReadablePeriod instances are accepted.


Note that a period of 1 day is not equal to a period of 24 hours,
nor is 1 hour equal to 60 minutes. Only periods with the same amount
in each field are equal.


This is because periods represent an abstracted definition of a time
period (eg. a day may not actually be 24 hours, it might be 23 or 25
at daylight savings boundary).

 To compare the actual duration of two periods, convert both to Durations, an operation that emphasises that the result may differ according to the date you choose. 
Params:
  • readablePeriod –  a readable period to check against
Returns:true if all the field values and types are equal, false if
 not or the period is null or of an incorrect type
/**
     * Compares this object with the specified object for equality based
     * on the value and type of each supported field.
     * All ReadablePeriod instances are accepted.
     * <p>
     * Note that a period of 1 day is not equal to a period of 24 hours,
     * nor is 1 hour equal to 60 minutes. Only periods with the same amount
     * in each field are equal.
     * <p>
     * This is because periods represent an abstracted definition of a time
     * period (eg. a day may not actually be 24 hours, it might be 23 or 25
     * at daylight savings boundary).
     * <p>
     * To compare the actual duration of two periods, convert both to
     * {@link Duration}s, an operation that emphasises that the result may
     * differ according to the date you choose.
     *
     * @param readablePeriod  a readable period to check against
     * @return true if all the field values and types are equal, false if
     *  not or the period is null or of an incorrect type
     */
    boolean equals(Object readablePeriod);

    
Gets a hash code for the period that is compatible with the equals method.
The hashcode is calculated as follows:

 int total = 17;
 for (int i = 0; i < fields.length; i++) {
     total = 27 * total + getValue(i);
     total = 27 * total + getFieldType(i).hashCode();
 }
 return total;



Returns:a hash code
/**
     * Gets a hash code for the period that is compatible with the equals method.
     * The hashcode is calculated as follows:
     * <pre>
     *  int total = 17;
     *  for (int i = 0; i < fields.length; i++) {
     *      total = 27 * total + getValue(i);
     *      total = 27 * total + getFieldType(i).hashCode();
     *  }
     *  return total;
     * </pre>
     *
     * @return a hash code
     */
    int hashCode();

    //-----------------------------------------------------------------------
    
Gets the value as a String in the style of the ISO8601 duration format.
Technically, the output can breach the ISO specification as weeks may be included.


For example, "PT6H3M5S" represents 6 hours, 3 minutes, 5 seconds.

Returns:the value as an ISO8601 style string
/**
     * Gets the value as a String in the style of the ISO8601 duration format.
     * Technically, the output can breach the ISO specification as weeks may be included.
     * <p>
     * For example, "PT6H3M5S" represents 6 hours, 3 minutes, 5 seconds.
     *
     * @return the value as an ISO8601 style string
     */
    String toString();

}