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  /*
   * This file is available under and governed by the GNU General Public
   * License version 2 only, as published by the Free Software Foundation.
   * However, the following notice accompanied the original version of this
   * file:
   *
   * Copyright (c) 2012, 2013 Stephen Colebourne & Michael Nascimento Santos
   *
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are met:
   *
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   *    this list of conditions and the following disclaimer.
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   *    without specific prior written permission.
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  package java.time.temporal;
  
  import java.time.DateTimeException;
  import java.time.Duration;
  import java.time.Period;
  import java.util.List;
  
  /**
   * Framework-level interface defining an amount of time, such as
   * "6 hours", "8 days" or "2 years and 3 months".
   * <p>
   * This is the base interface type for amounts of time.
   * An amount is distinct from a date or time-of-day in that it is not tied
   * to any specific point on the time-line.
   * <p>
   * The amount can be thought of as a {@code Map} of {@link TemporalUnit} to
   * {@code long}, exposed via {@link #getUnits()} and {@link #get(TemporalUnit)}.
   * A simple case might have a single unit-value pair, such as "6 hours".
   * A more complex case may have multiple unit-value pairs, such as
   * "7 years, 3 months and 5 days".
   * <p>
   * There are two common implementations.
   * {@link Period} is a date-based implementation, storing years, months and days.
   * {@link Duration} is a time-based implementation, storing seconds and nanoseconds,
   * but providing some access using other duration based units such as minutes,
   * hours and fixed 24-hour days.
   * <p>
   * This interface is a framework-level interface that should not be widely
   * used in application code. Instead, applications should create and pass
   * around instances of concrete types, such as {@code Period} and {@code Duration}.
   *
   * @implSpec
   * This interface places no restrictions on the mutability of implementations,
   * however immutability is strongly recommended.
   *
   * @since 1.8
   */
  public interface TemporalAmount {
  
      /**
       * Returns the value of the requested unit.
       * The units returned from {@link #getUnits()} uniquely define the
       * value of the {@code TemporalAmount}.  A value must be returned
       * for each unit listed in {@code getUnits}.
       *
       * @implSpec
       * Implementations may declare support for units not listed by {@link #getUnits()}.
       * Typically, the implementation would define additional units
       * as conversions for the convenience of developers.
       *
       * @param unit the {@code TemporalUnit} for which to return the value
       * @return the long value of the unit
       * @throws DateTimeException if a value for the unit cannot be obtained
       * @throws UnsupportedTemporalTypeException if the {@code unit} is not supported
       */
      long get(TemporalUnit unit);
  
      /**
       * Returns the list of units uniquely defining the value of this TemporalAmount.
       * The list of {@code TemporalUnits} is defined by the implementation class.
       * The list is a snapshot of the units at the time {@code getUnits}
       * is called and is not mutable.
       * The units are ordered from longest duration to the shortest duration
       * of the unit.
       *
       * @implSpec
       * The list of units completely and uniquely represents the
       * state of the object without omissions, overlaps or duplication.
       * The units are in order from longest duration to shortest.
       *
       * @return the List of {@code TemporalUnits}; not null
       */
      List<TemporalUnit> getUnits();
  
      /**
       * Adds to the specified temporal object.
       * <p>
       * Adds the amount to the specified temporal object using the logic
       * encapsulated in the implementing class.
       * <p>
       * There are two equivalent ways of using this method.
       * The first is to invoke this method directly.
       * The second is to use {@link Temporal#plus(TemporalAmount)}:
       * <pre>
       *   // These two lines are equivalent, but the second approach is recommended
       *   dateTime = amount.addTo(dateTime);
       *   dateTime = dateTime.plus(adder);
       * </pre>
       * It is recommended to use the second approach, {@code plus(TemporalAmount)},
       * as it is a lot clearer to read in code.
       *
       * @implSpec
       * The implementation must take the input object and add to it.
       * The implementation defines the logic of the addition and is responsible for
       * documenting that logic. It may use any method on {@code Temporal} to
       * query the temporal object and perform the addition.
       * The returned object must have the same observable type as the input object
       * <p>
       * The input object must not be altered.
       * Instead, an adjusted copy of the original must be returned.
       * This provides equivalent, safe behavior for immutable and mutable temporal objects.
       * <p>
       * The input temporal object may be in a calendar system other than ISO.
       * Implementations may choose to document compatibility with other calendar systems,
       * or reject non-ISO temporal objects by {@link TemporalQueries#chronology() querying the chronology}.
       * <p>
       * This method may be called from multiple threads in parallel.
       * It must be thread-safe when invoked.
       *
       * @param temporal  the temporal object to add the amount to, not null
       * @return an object of the same observable type with the addition made, not null
       * @throws DateTimeException if unable to add
       * @throws ArithmeticException if numeric overflow occurs
       */
      Temporal addTo(Temporal temporal);
  
      /**
       * Subtracts this object from the specified temporal object.
       * <p>
       * Subtracts the amount from the specified temporal object using the logic
       * encapsulated in the implementing class.
       * <p>
       * There are two equivalent ways of using this method.
       * The first is to invoke this method directly.
       * The second is to use {@link Temporal#minus(TemporalAmount)}:
       * <pre>
       *   // these two lines are equivalent, but the second approach is recommended
       *   dateTime = amount.subtractFrom(dateTime);
       *   dateTime = dateTime.minus(amount);
       * </pre>
       * It is recommended to use the second approach, {@code minus(TemporalAmount)},
       * as it is a lot clearer to read in code.
       *
       * @implSpec
       * The implementation must take the input object and subtract from it.
       * The implementation defines the logic of the subtraction and is responsible for
       * documenting that logic. It may use any method on {@code Temporal} to
       * query the temporal object and perform the subtraction.
       * The returned object must have the same observable type as the input object
       * <p>
       * The input object must not be altered.
       * Instead, an adjusted copy of the original must be returned.
       * This provides equivalent, safe behavior for immutable and mutable temporal objects.
       * <p>
       * The input temporal object may be in a calendar system other than ISO.
       * Implementations may choose to document compatibility with other calendar systems,
       * or reject non-ISO temporal objects by {@link TemporalQueries#chronology() querying the chronology}.
       * <p>
       * This method may be called from multiple threads in parallel.
       * It must be thread-safe when invoked.
       *
       * @param temporal  the temporal object to subtract the amount from, not null
       * @return an object of the same observable type with the subtraction made, not null
       * @throws DateTimeException if unable to subtract
       * @throws ArithmeticException if numeric overflow occurs
       */
      Temporal subtractFrom(Temporal temporal);
  }
  