ITEM14. Comparable을 구현할지 고려하라

https://codingwell.tistory.com/96

compareTo는 Object의 메서드가 아니다

두가지 성격만 빼면 Object의 equals와 같다

• 단순 동치성 비교에 더해 순서까지 비교할 수 있으며, 제네릭하다.
• Comparable을 구현했다는 것은 그 클래스의 인스턴스들에 자연적인 순서(natural order)가 있음을 뜻한다.
  그래서  Comparable을 구현한 객체들의 배열은 손쉽게 정렬할 수 있다.
  Array.sort(a);

 

---

규약

순서를 비교할 때 이 객체가 주어진 객체보다 작으면 음의정수(-1), 같으면 (0), 크면 양의정수(+1) 반환

비교할 수 없는 타입의 객체가 주어지면 ClassCastException 을 던짐

 

1) x.compareTo(y) == -y.compareTo(x)

두 객체 참조의 순서를 바꿔 비교해도 예상한 결과가 나와야함

 

2) 추이성 (x.compareTo(y) > 0 && y.compareTo(z) > 0) == x.compareTo(z) > 0

두 번째 규약은 첫번째가 두 번째보다 크고 두 번째가 세 번째보다 크다면, 첫번째는 세번째보다 커야한다.

 

3) x.compareTo(y) == 0 == (x.compareTo(z) == y.compareTo(z))

크기가 같은 객체들끼리는 어떤 객체와 비교하더라도 항상 같아야 한다는 뜻이다.

 

4) (x.compareTo(y) == 0) == x.eqauls(y)

이 권고는 필수가 아니지만, 꼭 지키는것이 좋다. (혹시 지키지 못하면 명시해줘야 한다.)

지키지 않는다면 컬렉션에 넣으면 해당 컬렉션이 구현한 인터페이스에 정의한 동작과 엇박자를 낼 것이다.

정렬된 컬렉션(TreeSet 등)은 동치성을 비교할때 equals가 아닌 compareTo를 사용하기 때문이다.

void example() {
    BigDecimal number1 = new BigDecimal("1.0");
    BigDecimal number2 = new BigDecimal("1.00");

    Set<BigDecimal> set1 = new HashSet<>();
    set1.add(number1);
    set1.add(number2);  //set1은 equals()로 비교하므로 원소가 2개

    Set<BigDecimal> set2 = new TreeSet<>();
    set2.add(number1);
    set2.add(number2);  //set2는 compareTo()로 비교하므로 원소가 1개
}

 

---

compareTo() 작성 요령

• Comparable은 타입을 인수로 받는 제네릭 인터페이스 → compareTo() 인수 타입은 컴파일타임에 정해짐
• compareTo()는 각 필드가 동치인지를 비교하는것이 아니라 순서를 비교
• Comparable을 구현하지 않은 필드나 표준이 아닌 순서로 비교해야한다면 Comparator(비교자)를 대신 사용 (비교자는 직접 만들거나 자바가 제공하는 것 골라 쓰기)
• compareTo()에서는 관계연산자 <,>를 사용하는 이전 방식은 거추장스럽고 오류를 유발하므로 박싱 기본 타입 클래스들의 정적 메서드 compare()을 사용하자.
• 핵심 필드부터 비교해나가서 순서가 먼저 결정되면 곧장 반환하자.

    /**
     * Returns 0 if the arguments are identical and {@code
     * c.compare(a, b)} otherwise.
     * Consequently, if both arguments are {@code null} 0
     * is returned.
     *
     * <p>Note that if one of the arguments is {@code null}, a {@code
     * NullPointerException} may or may not be thrown depending on
     * what ordering policy, if any, the {@link Comparator Comparator}
     * chooses to have for {@code null} values.
     *
     * @param <T> the type of the objects being compared
     * @param a an object
     * @param b an object to be compared with {@code a}
     * @param c the {@code Comparator} to compare the first two arguments
     * @return 0 if the arguments are identical and {@code
     * c.compare(a, b)} otherwise.
     * @see Comparable
     * @see Comparator
     */
    public static <T> int compare(T a, T b, Comparator<? super T> c) {
        return (a == b) ? 0 :  c.compare(a, b);
    }

/*
 * Copyright (c) 1997, 2014, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */

package java.util;

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

/**
 * 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 <tt>c</tt> on a set of elements
 * <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
 * <tt>c.compare(e1, e2)==0</tt> has the same boolean value as
 * <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
 * <tt>S</tt>.<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 <tt>c</tt>
 * is used with elements (or keys) drawn from a set <tt>S</tt>.  If the
 * ordering imposed by <tt>c</tt> on <tt>S</tt> 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 <tt>equals</tt>.<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
 * <tt>java.io.Serializable</tt>, 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 <tt>Serializable</tt>.<p>
 *
 * For the mathematically inclined, the <i>relation</i> that defines the
 * <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a
 * given set of objects <tt>S</tt> 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 <tt>compare</tt> that the
 * quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
 * imposed ordering is a <i>total order</i> on <tt>S</tt>.  When we say that
 * the ordering imposed by <tt>c</tt> on <tt>S</tt> 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}/../technotes/guides/collections/index.html">
 * 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
 */
@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>
     *
     * In the foregoing description, the notation
     * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
     * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
     * <tt>0</tt>, or <tt>1</tt> according to whether the value of
     * <i>expression</i> is negative, zero or positive.<p>
     *
     * The implementor must ensure that <tt>sgn(compare(x, y)) ==
     * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
     * implies that <tt>compare(x, y)</tt> must throw an exception if and only
     * if <tt>compare(y, x)</tt> throws an exception.)<p>
     *
     * The implementor must also ensure that the relation is transitive:
     * <tt>((compare(x, y)&gt;0) &amp;&amp; (compare(y, z)&gt;0))</tt> implies
     * <tt>compare(x, z)&gt;0</tt>.<p>
     *
     * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>
     * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
     * <tt>z</tt>.<p>
     *
     * It is generally the case, but <i>not</i> strictly required that
     * <tt>(compare(x, y)==0) == (x.equals(y))</tt>.  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."
     *
     * @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.
     */
    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
     * <tt>true</tt> <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)</code> implies that <tt>sgn(comp1.compare(o1,
     * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
     * <tt>o1</tt> and <tt>o2</tt>.<p>
     *
     * Note that it is <i>always</i> safe <i>not</i> to override
     * <tt>Object.equals(Object)</tt>.  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</code> 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()
     */
    boolean equals(Object obj);

    /**
     * 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
     */
    default Comparator<T> reversed() {
        return Collections.reverseOrder(this);
    }

    /**
     * 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.
     *
     * @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>
     *
     * @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.
     * @since 1.8
     */
    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}.
     *
     * @implSpec This default implementation behaves as if {@code
     *           thenComparing(comparing(keyExtractor, cmp))}.
     *
     * @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.
     * @see #comparing(Function, Comparator)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    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.
     *
     * @implSpec This default implementation behaves as if {@code
     *           thenComparing(comparing(keyExtractor))}.
     *
     * @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.
     * @see #comparing(Function)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    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 a {@code int} sort key.
     *
     * @implSpec This default implementation behaves as if {@code
     *           thenComparing(comparingInt(keyExtractor))}.
     *
     * @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.
     * @see #comparingInt(ToIntFunction)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    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.
     *
     * @implSpec This default implementation behaves as if {@code
     *           thenComparing(comparingLong(keyExtractor))}.
     *
     * @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.
     * @see #comparingLong(ToLongFunction)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    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.
     *
     * @implSpec This default implementation behaves as if {@code
     *           thenComparing(comparingDouble(keyExtractor))}.
     *
     * @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.
     * @see #comparingDouble(ToDoubleFunction)
     * @see #thenComparing(Comparator)
     * @since 1.8
     */
    default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) {
        return thenComparing(comparingDouble(keyExtractor));
    }

    /**
     * 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
     */
    public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
        return Collections.reverseOrder();
    }

    /**
     * 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
     */
    @SuppressWarnings("unchecked")
    public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {
        return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
    }

    /**
     * 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
     */
    public 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
     */
    public 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.
     *
     * @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>
     *
     * @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
     * @since 1.8
     */
    public 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.
     *
     * @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>
     *
     * @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
     * @since 1.8
     */
    public 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
     * @see #comparing(Function)
     * @throws NullPointerException if the argument is null
     * @since 1.8
     */
    public 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
     * @see #comparing(Function)
     * @throws NullPointerException if the argument is null
     * @since 1.8
     */
    public 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
     * @see #comparing(Function)
     * @throws NullPointerException if the argument is null
     * @since 1.8
     */
    public 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));
    }
}

---

비교자 생성 메서드를 이용한 비교자

자바8에서 Comparator 인터페이스가 비교자 생성 메서드를 이용하여 비교자를 생성할 수 있게 되었다.

객체 비교가 복잡해지는 경우에 Comparator 비교자를 활용할 수 있다.

체이닝하여 순차적으로 필드를 비교할 수 있다.

private static final Comparator<PhoneNumber> COMPARATOR =
            comparingInt((PhoneNumber pn) -> pn.areaCode)
                    .thenComparingInt(pn -> pn.prefix)
                    .thenComparingInt(pn -> pn.lineNum);

@Override
public int compareTo(PhoneNumber pn) {
    return COMPARATOR.compare(this, pn);
}

 

---

주의사항

//추이성 위배
static Comparator<Object> hashCodeOrder = new Comparator<>() {
    public int compare(Object o1, Object o2) {
        return o1.hashCode() - o2.hashCode();
    }
};

값의 차를 기준으로 비교하는 위의 코드는 사용하면 안된다.

이 방식은 정수 오버플로우(컴퓨터의 정수 연산의 계산 결과가 허용 범위를 초과할 때 발생하는 오류)나

부동 소수점 계산 방식(떠돌이 소수점[1] 방식은 실수를 컴퓨터상에서 근사하여 표현할 때 소수점의 위치를 고정하지 않고 그 위치를 나타내는 수를 따로 적는 것)에 따른 오류를 낼 수도 있다.

대신 아래의 방식 중 하나를 사용하자.

static Comparator<Object> hashCodeOrder = new Comparator<>() {
    public int compare(Object o1, Object o2) {
        return Integer.compare(o1.hashCode(), o2.hashCode());
    }
};

static Comparator<Object> hashCodeOrder = Comparator.comparingInt(o -> o.hashCode());

---

결론

순서를 고려해야하는 값 클래스를 작성하게 된다면 꼭 Comparable 인터페이스를 구현하여

인스턴스들을 쉽게 정렬/검색하고 비교 기능을 제공하는 컬렉션과 어우러져야 한다.

compareTo()에서는 필드 값 비교시에 <,>를 사용하지 말고 박싱된 기본 타입 클래스가 제공하는 정적 메서드인

compare()이나 Comparator 인터페이스가 제공하는 비교자 생성 메서드를 사용하도록 한다.