[数据结构与算法]java compareTo 方法 和 Comparator 接口

1. compareTo 方法是接口 Comparable 的实现

返回值是整型，它是先比较对应字符的大小(ASCII码顺序)，如果第一个字符和参数的第一个字符不等，结束比较，返回他们之间的长度差值，如果第一个字符和参数的第一个字符相等，则以第二个字符和参数的第二个字符做比较，以此类推,直至比较的字符或被比较的字符有一方结束。

• 如果参数字符串等于此字符串，则返回值 0；
• 如果此字符串小于字符串参数，则返回一个小于 0 的值；
• 如果此字符串大于字符串参数，则返回一个大于 0 的值。

说明：

如果第一个字符和参数的第一个字符不等，结束比较，返回第一个字符的ASCII码差值。

如果第一个字符和参数的第一个字符相等，则以第二个字符和参数的第二个字符做比较，以此类推,直至不等为止，返回该字符的ASCII码差值。 如果两个字符串不一样长，可对应字符又完全一样，则返回两个字符串的长度差值。

Integer 和 String 实现了compareTo 方法

实体对象实现方法

package com.study.stream;

import java.util.Objects;

public class Person implements Comparable<Person>{
    private String name;
    private Integer age;
    private Integer height;

    public Person(String name, Integer age, Integer height) {
        this.name = name;
        this.age = age;
        this.height = height;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Integer getHeight() {
        return height;
    }

    public void setHeight(Integer height) {
        this.height = height;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Person person = (Person) o;
        return name.equals(person.name) &&
                age.equals(person.age);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

    @Override
    public String toString() {
        return String.format("姓名:%s;年龄:%s;身高:%s;",this.name,this.age,this.height);
    }


    /**
     * Compares this object with the specified object for order.  Returns a
     * negative integer, zero, or a positive integer as this object is less
     * than, equal to, or greater than the specified object.
     *
     * <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==
     * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
     * implies that <tt>x.compareTo(y)</tt> must throw an exception iff
     * <tt>y.compareTo(x)</tt> throws an exception.)
     *
     * <p>The implementor must also ensure that the relation is transitive:
     * <tt>(x.compareTo(y)&gt;0 &amp;&amp; y.compareTo(z)&gt;0)</tt> implies
     * <tt>x.compareTo(z)&gt;0</tt>.
     *
     * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>
     * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for
     * all <tt>z</tt>.
     *
     * <p>It is strongly recommended, but <i>not</i> strictly required that
     * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>.  Generally speaking, any
     * class that implements the <tt>Comparable</tt> interface and violates
     * this condition should clearly indicate this fact.  The recommended
     * language is "Note: this class has a natural ordering that is
     * inconsistent with equals."
     *
     * <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.
     *
     * @param o the object to be compared.
     * @return a negative integer, zero, or a positive integer as this object
     * is less than, equal to, or greater than the specified object.
     * @throws NullPointerException if the specified object is null
     * @throws ClassCastException   if the specified object's type prevents it
     *                              from being compared to this object.
     */
    @Override
    public int compareTo(Person o) {
        return this.getAge().compareTo(o.getAge());
    }
}

实体对象排序:

2.Comparator 接口

Java提供了一个用于比较的接口Comparable,提供了一个比较的方法,所有实现该接口的类,都动态的实现了该比较方法。实际上Java中除了比较一个接口外，还提供了一个接口，该接口也是具有比较的功能，但该接口注重的却是比较容器，然后对其排序，这就是Comparator

源码：

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;
 
@FunctionalInterface
public interface Comparator<T> {
 
    int compare(T o1, T o2);
 
    boolean equals(Object obj);
 
 
    default Comparator<T> reversed() {
        return Collections.reverseOrder(this);
    }
 
    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);
        };
    }
 
    default <U> Comparator<T> thenComparing(
            Function<? super T, ? extends U> keyExtractor,
            Comparator<? super U> keyComparator)
    {
        return thenComparing(comparing(keyExtractor, keyComparator));
    }
 
    default <U extends Comparable<? super U>> Comparator<T> thenComparing(
            Function<? super T, ? extends U> keyExtractor)
    {
        return thenComparing(comparing(keyExtractor));
    }
 
    default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor) {
        return thenComparing(comparingInt(keyExtractor));
    }
 
    default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor) {
        return thenComparing(comparingLong(keyExtractor));
    }
 
    default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) {
        return thenComparing(comparingDouble(keyExtractor));
    }
    public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
        return Collections.reverseOrder();
    }
 
    @SuppressWarnings("unchecked")
    public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {
        return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
    }
 
    public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {
        return new Comparators.NullComparator<>(true, comparator);
    }
 
    public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) {
        return new Comparators.NullComparator<>(false, comparator);
    }
 
    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));
    }
 
    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));
    }
 
 
    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));
    }
 
    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));
    }
 
 
    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));
    }
}

案例：

List list; 代表某集合

//返回 对象集合以类属性一升序排序

list.stream().sorted(Comparator.comparing(类::属性一));

//返回 对象集合以类属性一降序排序 注意两种写法

list.stream().sorted(Comparator.comparing(类::属性一).reversed());

//先以属性一升序,结果进行属性一降序

list.stream().sorted(Comparator.comparing(类::属性一,Comparator.reverseOrder()));

//以属性一降序

//返回 对象集合以类属性一升序 属性二升序

list.stream().sorted(Comparator.comparing(类::属性一).thenComparing(类::属性二));

//返回 对象集合以类属性一降序 属性二升序 注意两种写法

list.stream().sorted(Comparator.comparing(类::属性一).reversed().thenComparing(类::属性二));

//先以属性一升序,升序结果进行属性一降序,再进行属性二升序

list.stream().sorted(Comparator.comparing(类::属性一,Comparator.reverseOrder()).thenComparing(类::属性二));

//先以属性一降序,再进行属性二升序

//返回 对象集合以类属性一降序 属性二降序 注意两种写法

list.stream().sorted(Comparator.comparing(类::属性一).reversed().thenComparing(类::属性二,Comparator.reverseOrder()));

//先以属性一升序,升序结果进行属性一降序,再进行属性二降序

list.stream().sorted(Comparator.comparing(类::属性一,Comparator.reverseOrder()).thenComparing(类::属性二,Comparator.reverseOrder()));

//先以属性一降序,再进行属性二降序

//返回 对象集合以类属性一升序 属性二降序 注意两种写法

list.stream().sorted(Comparator.comparing(类::属性一).reversed().thenComparing(类::属性二).reversed());

//先以属性一升序,升序结果进行属性一降序,再进行属性二升序,结果进行属性一降序属性二降序

list.stream().sorted(Comparator.comparing(类::属性一).thenComparing(类::属性二,Comparator.reverseOrder()));

//先以属性一升序,再进行属性二降序

通过以上例子我们可以发现

1. Comparator.comparing(类::属性一).reversed();

2. Comparator.comparing(类::属性一,Comparator.reverseOrder());

两种排序是完全不一样的,一定要区分开来 1 是得到排序结果后再排序,2是直接进行排序,很多人会混淆导致理解出错,2更好理解,建议使用

代码：

System.out.println("***************--sorted 定义排序规则 身高,年龄排序--***************************");
personList.stream().sorted(Comparator.comparing(Person::getHeight,Comparator.naturalOrder()).thenComparing(Person::getAge,Comparator.naturalOrder())).forEach(System.out::println);