TemporalAmount.java

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 * Copyright (c) 2012, 2013 Stephen Colebourne & Michael Nascimento Santos
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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
 */
// "时间段"的基础接口，主要用来查看"时间段"的结构，以及将当前"时间段"与某个时间量进行增/减运算
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}.
     *
     * @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
     * @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.
     */
    // 返回当前"时间段"中指定的时间量单位unit对应的时间量数值
    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.
     *
     * @return the List of {@code TemporalUnits}; not null
     *
     * @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.
     */
    // 返回当前"时间段"上可用的时间量单位，这其实是该"时间段"的组成部件
    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.
     *
     * @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
     * @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.
     */
    /*
     * 增加目标时间量temporal
     *
     * 尝试将当前"时间段"累加到指定的时间量temporal上，
     * 如果累加后的值与原值相同，则返回temporal自身；否则，会构造一个新对象再返回。
     */
    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.
     *
     * @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
     * @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.
     */
    /*
     * 减少目标时间量temporal
     *
     * 尝试从指定的时间量temporal上减去当前"时间段"，
     * 如果减少后的值与原值相同，则返回temporal自身；否则，会构造一个新对象再返回。
     */
    Temporal subtractFrom(Temporal temporal);
    
}