Comparator.java

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package java.util;

import java.io.Serializable;
import java.util.function.Function;
import java.util.function.ToDoubleFunction;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;

/**
 * A comparison function, which imposes a <i>total ordering</i> on some
 * collection of objects.  Comparators can be passed to a sort method (such
 * as {@link Collections#sort(List, Comparator) Collections.sort} or {@link
 * Arrays#sort(Object[], Comparator) Arrays.sort}) to allow precise control
 * over the sort order.  Comparators can also be used to control the order of
 * certain data structures (such as {@link SortedSet sorted sets} or {@link
 * SortedMap sorted maps}), or to provide an ordering for collections of
 * objects that don't have a {@link Comparable natural ordering}.<p>
 *
 * The ordering imposed by a comparator {@code c} on a set of elements
 * {@code S} is said to be <i>consistent with equals</i> if and only if
 * {@code c.compare(e1, e2)==0} has the same boolean value as
 * {@code e1.equals(e2)} for every {@code e1} and {@code e2} in
 * {@code S}.<p>
 *
 * Caution should be exercised when using a comparator capable of imposing an
 * ordering inconsistent with equals to order a sorted set (or sorted map).
 * Suppose a sorted set (or sorted map) with an explicit comparator {@code c}
 * is used with elements (or keys) drawn from a set {@code S}.  If the
 * ordering imposed by {@code c} on {@code S} is inconsistent with equals,
 * the sorted set (or sorted map) will behave "strangely."  In particular the
 * sorted set (or sorted map) will violate the general contract for set (or
 * map), which is defined in terms of {@code equals}.<p>
 *
 * For example, suppose one adds two elements {@code a} and {@code b} such that
 * {@code (a.equals(b) && c.compare(a, b) != 0)}
 * to an empty {@code TreeSet} with comparator {@code c}.
 * The second {@code add} operation will return
 * true (and the size of the tree set will increase) because {@code a} and
 * {@code b} are not equivalent from the tree set's perspective, even though
 * this is contrary to the specification of the
 * {@link Set#add Set.add} method.<p>
 *
 * Note: It is generally a good idea for comparators to also implement
 * {@code java.io.Serializable}, as they may be used as ordering methods in
 * serializable data structures (like {@link TreeSet}, {@link TreeMap}).  In
 * order for the data structure to serialize successfully, the comparator (if
 * provided) must implement {@code Serializable}.<p>
 *
 * For the mathematically inclined, the <i>relation</i> that defines the
 * <i>imposed ordering</i> that a given comparator {@code c} imposes on a
 * given set of objects {@code S} is:<pre>
 *       {(x, y) such that c.compare(x, y) &lt;= 0}.
 * </pre> The <i>quotient</i> for this total order is:<pre>
 *       {(x, y) such that c.compare(x, y) == 0}.
 * </pre>
 *
 * It follows immediately from the contract for {@code compare} that the
 * quotient is an <i>equivalence relation</i> on {@code S}, and that the
 * imposed ordering is a <i>total order</i> on {@code S}.  When we say that
 * the ordering imposed by {@code c} on {@code S} is <i>consistent with
 * equals</i>, we mean that the quotient for the ordering is the equivalence
 * relation defined by the objects' {@link Object#equals(Object)
 * equals(Object)} method(s):<pre>
 *     {(x, y) such that x.equals(y)}. </pre>
 *
 * <p>Unlike {@code Comparable}, a comparator may optionally permit
 * comparison of null arguments, while maintaining the requirements for
 * an equivalence relation.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @param <T> the type of objects that may be compared by this comparator
 *
 * @author Josh Bloch
 * @author Neal Gafter
 * @see Comparable
 * @see java.io.Serializable
 * @since 1.2
 */
/*
 * 外部比较器，支持以自定义的顺序来比较元素
 *
 * 外部比较器的特点是：可拔插
 * 其比较行为可以挂载到待比较对象的外部
 *
 * 该接口已函数化，如：
 * Comparator<Integer> comparator = new Comparator<Integer>() {
 *     @Override
 *     public int compare(Integer a, Integer b) {
 *         return a - b;
 *     }
 * };
 * 可以简写为：
 * Comparator<Integer> comparator = (a, b)->(a-b);
 *
 * 注：区别于内部比较器Comparable
 */
@FunctionalInterface
public interface Comparator<T> {
    
    /**
     * Compares its two arguments for order.  Returns a negative integer,
     * zero, or a positive integer as the first argument is less than, equal
     * to, or greater than the second.<p>
     *
     * The implementor must ensure that {@code sgn(compare(x, y)) ==
     * -sgn(compare(y, x))} for all {@code x} and {@code y}.  (This
     * implies that {@code compare(x, y)} must throw an exception if and only
     * if {@code compare(y, x)} throws an exception.)<p>
     *
     * The implementor must also ensure that the relation is transitive:
     * {@code ((compare(x, y)>0) && (compare(y, z)>0))} implies
     * {@code compare(x, z)>0}.<p>
     *
     * Finally, the implementor must ensure that {@code compare(x, y)==0}
     * implies that {@code sgn(compare(x, z))==sgn(compare(y, z))} for all
     * {@code z}.<p>
     *
     * It is generally the case, but <i>not</i> strictly required that
     * {@code (compare(x, y)==0) == (x.equals(y))}.  Generally speaking,
     * any comparator that violates this condition should clearly indicate
     * this fact.  The recommended language is "Note: this comparator
     * imposes orderings that are inconsistent with equals."<p>
     *
     * In the foregoing description, the notation
     * {@code sgn(}<i>expression</i>{@code )} designates the mathematical
     * <i>signum</i> function, which is defined to return one of {@code -1},
     * {@code 0}, or {@code 1} according to whether the value of
     * <i>expression</i> is negative, zero, or positive, respectively.
     *
     * @param o1 the first object to be compared.
     * @param o2 the second object to be compared.
     *
     * @return a negative integer, zero, or a positive integer as the
     * first argument is less than, equal to, or greater than the
     * second.
     *
     * @throws NullPointerException if an argument is null and this
     *                              comparator does not permit null arguments
     * @throws ClassCastException   if the arguments' types prevent them from
     *                              being compared by this comparator.
     */
    // 比较两个对象，返回负数、0、正数，分别代表o1<o2、o1==o2、o1>o2
    int compare(T o1, T o2);
    
    /**
     * Indicates whether some other object is &quot;equal to&quot; this
     * comparator.  This method must obey the general contract of
     * {@link Object#equals(Object)}.  Additionally, this method can return
     * {@code true} <i>only</i> if the specified object is also a comparator
     * and it imposes the same ordering as this comparator.  Thus,
     * {@code comp1.equals(comp2)} implies that {@code sgn(comp1.compare(o1,
     * o2))==sgn(comp2.compare(o1, o2))} for every object reference
     * {@code o1} and {@code o2}.<p>
     *
     * Note that it is <i>always</i> safe <i>not</i> to override
     * {@code Object.equals(Object)}.  However, overriding this method may,
     * in some cases, improve performance by allowing programs to determine
     * that two distinct comparators impose the same order.
     *
     * @param obj the reference object with which to compare.
     *
     * @return {@code true} only if the specified object is also
     * a comparator and it imposes the same ordering as this
     * comparator.
     *
     * @see Object#equals(Object)
     * @see Object#hashCode()
     */
    // 比较两个对象是否相等
    boolean equals(Object obj);
    
    /**
     * Returns a comparator that compares {@link Comparable} objects in natural order.
     *
     * <p>The returned comparator is serializable and throws {@link
     * NullPointerException} when comparing {@code null}.
     *
     * @param <T> the {@link Comparable} type of element to be compared
     *
     * @return a comparator that imposes the <i>natural ordering</i> on {@code Comparable} objects.
     *
     * @see Comparable
     * @since 1.8
     */
    /*
     * 返回“自然顺序”比较器，用于比较实现了Comparable的对象
     *
     * 注：所谓自然顺序是指顺应对象内部的Comparable排序规则
     */
    @SuppressWarnings("unchecked")
    static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {
        return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
    }
    
    /**
     * Returns a comparator that imposes the reverse of the <em>natural ordering</em>.
     *
     * <p>The returned comparator is serializable and throws {@link
     * NullPointerException} when comparing {@code null}.
     *
     * @param <T> the {@link Comparable} type of element to be compared
     *
     * @return a comparator that imposes the reverse of the <i>natural
     * ordering</i> on {@code Comparable} objects.
     *
     * @see Comparable
     * @since 1.8
     */
    /*
     * 返回“逆自然顺序”比较器，用于比较实现了Comparable的对象
     *
     * 注：所谓自然顺序是指顺应对象内部的Comparable排序规则
     */
    static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
        return Collections.reverseOrder();
    }
    
    /**
     * Returns a comparator that imposes the reverse ordering of this comparator.
     *
     * @return a comparator that imposes the reverse ordering of this
     * comparator.
     *
     * @since 1.8
     */
    // 返回一个比较器，它强制执行与当前比较器相反的顺序。
    default Comparator<T> reversed() {
        return Collections.reverseOrder(this);
    }
    
    /**
     * Returns a null-friendly comparator that considers {@code null} to be
     * less than non-null. When both are {@code null}, they are considered
     * equal. If both are non-null, the specified {@code Comparator} is used
     * to determine the order. If the specified comparator is {@code null},
     * then the returned comparator considers all non-null values to be equal.
     *
     * <p>The returned comparator is serializable if the specified comparator
     * is serializable.
     *
     * @param <T>        the type of the elements to be compared
     * @param comparator a {@code Comparator} for comparing non-null values
     *
     * @return a comparator that considers {@code null} to be less than
     * non-null, and compares non-null objects with the supplied
     * {@code Comparator}.
     *
     * @since 1.8
     */
    // 返回nullFirst比较器，且nullFirst==true，即null被认为是序列中第一个元素
    static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {
        return new Comparators.NullComparator<>(true, comparator);
    }
    
    /**
     * Returns a null-friendly comparator that considers {@code null} to be
     * greater than non-null. When both are {@code null}, they are considered
     * equal. If both are non-null, the specified {@code Comparator} is used
     * to determine the order. If the specified comparator is {@code null},
     * then the returned comparator considers all non-null values to be equal.
     *
     * <p>The returned comparator is serializable if the specified comparator
     * is serializable.
     *
     * @param <T>        the type of the elements to be compared
     * @param comparator a {@code Comparator} for comparing non-null values
     *
     * @return a comparator that considers {@code null} to be greater than
     * non-null, and compares non-null objects with the supplied
     * {@code Comparator}.
     *
     * @since 1.8
     */
    // 返回nullFirst比较器，且nullFirst==false，即null被认为是序列中最后一个元素
    static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) {
        return new Comparators.NullComparator<>(false, comparator);
    }
    
    /**
     * Accepts a function that extracts a sort key from a type {@code T}, and
     * returns a {@code Comparator<T>} that compares by that sort key using
     * the specified {@link Comparator}.
     *
     * <p>The returned comparator is serializable if the specified function
     * and comparator are both serializable.
     *
     * @param <T>           the type of element to be compared
     * @param <U>           the type of the sort key
     * @param keyExtractor  the function used to extract the sort key
     * @param keyComparator the {@code Comparator} used to compare the sort key
     *
     * @return a comparator that compares by an extracted key using the
     * specified {@code Comparator}
     *
     * @throws NullPointerException if either argument is null
     * @apiNote For example, to obtain a {@code Comparator} that compares {@code
     * Person} objects by their last name ignoring case differences,
     *
     * <pre>{@code
     *     Comparator<Person> cmp = Comparator.comparing(
     *             Person::getLastName,
     *             String.CASE_INSENSITIVE_ORDER);
     * }</pre>
     * @since 1.8
     */
    /*
     * 1.使用keyExtractor处理待比较元素
     * 2.使用keyComparator比较处理后的元素
     */
    static <T, U> Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator) {
        Objects.requireNonNull(keyExtractor);
        Objects.requireNonNull(keyComparator);
        return (Comparator<T> & Serializable) (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1), keyExtractor.apply(c2));
    }
    
    /**
     * Accepts a function that extracts a {@link java.lang.Comparable
     * Comparable} sort key from a type {@code T}, and returns a {@code
     * Comparator<T>} that compares by that sort key.
     *
     * <p>The returned comparator is serializable if the specified function
     * is also serializable.
     *
     * @param <T>          the type of element to be compared
     * @param <U>          the type of the {@code Comparable} sort key
     * @param keyExtractor the function used to extract the {@link
     *                     Comparable} sort key
     *
     * @return a comparator that compares by an extracted key
     *
     * @throws NullPointerException if the argument is null
     * @apiNote For example, to obtain a {@code Comparator} that compares {@code
     * Person} objects by their last name,
     *
     * <pre>{@code
     *     Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
     * }</pre>
     * @since 1.8
     */
    /*
     * 1.使用keyExtractor处理待比较元素
     * 2.使用compareTo()方法比较两个处理后的元素
     * -->注：要求处理后的元素实现Comparable接口
     */
    static <T, U extends Comparable<? super U>> Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor) {
        Objects.requireNonNull(keyExtractor);
        return (Comparator<T> & Serializable) (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
    }
    
    /**
     * Accepts a function that extracts an {@code int} sort key from a type
     * {@code T}, and returns a {@code Comparator<T>} that compares by that
     * sort key.
     *
     * <p>The returned comparator is serializable if the specified function
     * is also serializable.
     *
     * @param <T>          the type of element to be compared
     * @param keyExtractor the function used to extract the integer sort key
     *
     * @return a comparator that compares by an extracted key
     *
     * @throws NullPointerException if the argument is null
     * @see #comparing(Function)
     * @since 1.8
     */
    /*
     * 1.使用keyExtractor处理待比较元素
     * 2.使用Integer.compare()方法比较两个处理后的元素
     * -->注：要求处理后的元素兼容int类型
     */
    static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor) {
        Objects.requireNonNull(keyExtractor);
        return (Comparator<T> & Serializable) (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
    }
    
    /**
     * Accepts a function that extracts a {@code long} sort key from a type
     * {@code T}, and returns a {@code Comparator<T>} that compares by that
     * sort key.
     *
     * <p>The returned comparator is serializable if the specified function is
     * also serializable.
     *
     * @param <T>          the type of element to be compared
     * @param keyExtractor the function used to extract the long sort key
     *
     * @return a comparator that compares by an extracted key
     *
     * @throws NullPointerException if the argument is null
     * @see #comparing(Function)
     * @since 1.8
     */
    /*
     * 1.使用keyExtractor处理待比较元素
     * 2.使用Long.compare()方法比较两个处理后的元素
     * -->注：要求处理后的元素兼容long类型
     */
    static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor) {
        Objects.requireNonNull(keyExtractor);
        return (Comparator<T> & Serializable) (c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
    }
    
    /**
     * Accepts a function that extracts a {@code double} sort key from a type
     * {@code T}, and returns a {@code Comparator<T>} that compares by that
     * sort key.
     *
     * <p>The returned comparator is serializable if the specified function
     * is also serializable.
     *
     * @param <T>          the type of element to be compared
     * @param keyExtractor the function used to extract the double sort key
     *
     * @return a comparator that compares by an extracted key
     *
     * @throws NullPointerException if the argument is null
     * @see #comparing(Function)
     * @since 1.8
     */
    /*
     * 1.使用keyExtractor处理待比较元素
     * 2.使用Double.compare()方法比较两个处理后的元素
     * -->注：要求处理后的元素兼容double类型
     */
    static <T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor) {
        Objects.requireNonNull(keyExtractor);
        return (Comparator<T> & Serializable) (c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
    }
    
    /**
     * Returns a lexicographic-order comparator with another comparator.
     * If this {@code Comparator} considers two elements equal, i.e.
     * {@code compare(a, b) == 0}, {@code other} is used to determine the order.
     *
     * <p>The returned comparator is serializable if the specified comparator
     * is also serializable.
     *
     * @param other the other comparator to be used when this comparator
     *              compares two objects that are equal.
     *
     * @return a lexicographic-order comparator composed of this and then the
     * other comparator
     *
     * @throws NullPointerException if the argument is null.
     * @apiNote For example, to sort a collection of {@code String} based on the length
     * and then case-insensitive natural ordering, the comparator can be
     * composed using following code,
     *
     * <pre>{@code
     *     Comparator<String> cmp = Comparator.comparingInt(String::length)
     *             .thenComparing(String.CASE_INSENSITIVE_ORDER);
     * }</pre>
     * @since 1.8
     */
    /*
     * comparator.thenComparing(other)
     *
     * 1.使用comparator比较两个元素
     * 2.如果元素相同：
     *   2.1.使用other进一步比较两个元素
     */
    default Comparator<T> thenComparing(Comparator<? super T> other) {
        Objects.requireNonNull(other);
        
        return (Comparator<T> & Serializable) (c1, c2) -> {
            int res = compare(c1, c2);
            return (res != 0) ? res : other.compare(c1, c2);
        };
    }
    
    /**
     * Returns a lexicographic-order comparator with a function that
     * extracts a key to be compared with the given {@code Comparator}.
     *
     * @param <U>           the type of the sort key
     * @param keyExtractor  the function used to extract the sort key
     * @param keyComparator the {@code Comparator} used to compare the sort key
     *
     * @return a lexicographic-order comparator composed of this comparator
     * and then comparing on the key extracted by the keyExtractor function
     *
     * @throws NullPointerException if either argument is null.
     * @implSpec This default implementation behaves as if {@code
     * thenComparing(comparing(keyExtractor, cmp))}.
     * @see #comparing(Function, Comparator)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    /*
     * comparator.thenComparing(keyExtractor, keyComparator)
     *
     * 1.使用comparator比较两个元素
     * 2.如果元素相同：
     *   2.1.使用keyExtractor处理待比较元素
     *   2.2.使用keyComparator比较处理后的元素
     */
    default <U> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator) {
        return thenComparing(comparing(keyExtractor, keyComparator));
    }
    
    /**
     * Returns a lexicographic-order comparator with a function that
     * extracts a {@code Comparable} sort key.
     *
     * @param <U>          the type of the {@link Comparable} sort key
     * @param keyExtractor the function used to extract the {@link
     *                     Comparable} sort key
     *
     * @return a lexicographic-order comparator composed of this and then the
     * {@link Comparable} sort key.
     *
     * @throws NullPointerException if the argument is null.
     * @implSpec This default implementation behaves as if {@code
     * thenComparing(comparing(keyExtractor))}.
     * @see #comparing(Function)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    /*
     * comparator.thenComparing(keyExtractor)
     *
     * 1.使用comparator比较两个元素
     * 2.如果元素相同：
     *   2.1.使用keyExtractor处理待比较元素
     *   2.2.使用compareTo()方法比较两个处理后的元素
     *   -->注：要求处理后的元素实现Comparable接口
     */
    default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor) {
        return thenComparing(comparing(keyExtractor));
    }
    
    /**
     * Returns a lexicographic-order comparator with a function that
     * extracts an {@code int} sort key.
     *
     * @param keyExtractor the function used to extract the integer sort key
     *
     * @return a lexicographic-order comparator composed of this and then the
     * {@code int} sort key
     *
     * @throws NullPointerException if the argument is null.
     * @implSpec This default implementation behaves as if {@code
     * thenComparing(comparingInt(keyExtractor))}.
     * @see #comparingInt(ToIntFunction)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    /*
     * comparator.thenComparingInt(keyExtractor)
     *
     * 1.使用comparator比较两个元素
     * 2.如果元素相同：
     *   2.1.使用keyExtractor处理待比较元素
     *   2.2.使用Integer.compare()方法比较两个处理后的元素
     *   -->注：要求处理后的元素兼容int类型
     */
    default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor) {
        return thenComparing(comparingInt(keyExtractor));
    }
    
    /**
     * Returns a lexicographic-order comparator with a function that
     * extracts a {@code long} sort key.
     *
     * @param keyExtractor the function used to extract the long sort key
     *
     * @return a lexicographic-order comparator composed of this and then the
     * {@code long} sort key
     *
     * @throws NullPointerException if the argument is null.
     * @implSpec This default implementation behaves as if {@code
     * thenComparing(comparingLong(keyExtractor))}.
     * @see #comparingLong(ToLongFunction)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    /*
     * comparator.thenComparingLong(keyExtractor)
     *
     * 1.使用comparator比较两个元素
     * 2.如果元素相同：
     *   2.1.使用keyExtractor处理待比较元素
     *   2.2.使用Long.compare()方法比较两个处理后的元素
     *   -->注：要求处理后的元素兼容long类型
     */
    default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor) {
        return thenComparing(comparingLong(keyExtractor));
    }
    
    /**
     * Returns a lexicographic-order comparator with a function that
     * extracts a {@code double} sort key.
     *
     * @param keyExtractor the function used to extract the double sort key
     *
     * @return a lexicographic-order comparator composed of this and then the
     * {@code double} sort key
     *
     * @throws NullPointerException if the argument is null.
     * @implSpec This default implementation behaves as if {@code
     * thenComparing(comparingDouble(keyExtractor))}.
     * @see #comparingDouble(ToDoubleFunction)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    /*
     * comparator.thenComparingDouble(keyExtractor)
     *
     * 1.使用comparator比较两个元素
     * 2.如果元素相同：
     *   2.1.使用keyExtractor处理待比较元素
     *   2.2.使用Double.compare()方法比较两个处理后的元素
     *   -->注：要求处理后的元素兼容double类型
     */
    default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) {
        return thenComparing(comparingDouble(keyExtractor));
    }
}